diff --git a/sierra-previewer/build.gradle b/sierra-previewer/build.gradle
index 66b3321..3c813e0 100644
--- a/sierra-previewer/build.gradle
+++ b/sierra-previewer/build.gradle
@@ -13,6 +13,7 @@
  */
 
 plugins {
+    id 'java' 
     id 'application'
 }
 
@@ -24,6 +25,14 @@ dependencies {
 
     implementation 'com.fifesoft:rsyntaxtextarea:3.6.0'
     implementation 'com.fifesoft:autocomplete:3.3.2'
+
+    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.10.1'
+    testImplementation 'org.junit.jupiter:junit-jupiter-params:5.10.1'
+    testImplementation 'org.mockito:mockito-core:5.9.0'
+    testImplementation 'org.mockito:mockito-junit-jupiter:5.9.0'
+
+    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.10.1' 
+    testRuntimeOnly 'org.junit.platform:junit-platform-launcher'
 }
 
 tasks.withType(JavaCompile) {
@@ -31,3 +40,11 @@ tasks.withType(JavaCompile) {
         '-parameters'
     ]
 }
+
+tasks.named('test', Test) {
+    useJUnitPlatform()
+}
+
+application {
+    mainClass = 'org.httprpc.sierra.previewer.SierraPreviewerApp'
+}
diff --git a/sierra-previewer/gradle.properties b/sierra-previewer/gradle.properties
new file mode 100644
index 0000000..9569323
--- /dev/null
+++ b/sierra-previewer/gradle.properties
@@ -0,0 +1,2 @@
+action.custom-1=run
+action.custom-1.args=-w run
diff --git a/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/MainFrame.java b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/MainFrame.java
index bc4ca31..806d606 100644
--- a/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/MainFrame.java
+++ b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/MainFrame.java
@@ -11,9 +11,519 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 package org.httprpc.sierra.previewer;
 
-public class MainFrame {
-    // TODO
+import org.httprpc.sierra.previewer.engine.RenderingEngine;
+import org.httprpc.sierra.previewer.model.RenderError;
+import org.httprpc.sierra.previewer.model.RenderResult;
+import java.awt.*;
+import java.io.File;
+import java.io.IOException;
+import java.net.URL;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardOpenOption;
+import java.util.List;
+import java.util.concurrent.ExecutionException;
+import java.util.function.Consumer;
+import javax.swing.*;
+import javax.swing.event.DocumentEvent;
+import javax.swing.event.DocumentListener;
+import javax.swing.filechooser.FileNameExtensionFilter;
+
+import org.fife.ui.rsyntaxtextarea.RSyntaxTextArea;
+import org.fife.ui.rsyntaxtextarea.SyntaxConstants;
+
+import org.fife.ui.autocomplete.AutoCompletion;
+import org.fife.ui.autocomplete.CompletionProvider;
+
+import org.httprpc.sierra.UILoader;
+import org.httprpc.sierra.Outlet;
+
+/**
+ * The main application window for the Sierra UI Previewer. UI is defined in
+ * MainFrame.xml and loaded by UILoader. This class contains the wiring and
+ * business logic.
+ */
+public class MainFrame extends JFrame {
+
+    // --- Subsystems ---
+    private final RenderingEngine renderingEngine;
+    private final Timer debounceTimer;
+    private final RecentFilesManager recentFilesManager; // NEW: Manager instance
+
+    // --- File Handling State ---
+    private final JFileChooser fileChooser;
+    private Path currentFilePath = null;
+    // --- UI Components (Injected by Sierra) ---
+    // Stores the path of the currently loaded file
+    
+    @Outlet 
+    private JMenuBar menuBar;
+    
+    @Outlet
+    private JMenuItem openItem;    
+    
+    @Outlet
+    private JMenuItem saveItem;
+    
+    @Outlet
+    private JMenu recentMenu;
+    
+    @Outlet
+    private JMenuItem exitItem;          
+            
+    @Outlet
+    private JMenuItem aboutItem;    
+    
+    @Outlet
+    private JScrollPane editorScrollPane; // The <scroll-pane> placeholder
+
+    @Outlet
+    private JPanel previewPanel; // The <column-panel>
+
+    @Outlet
+    private JLabel statusBar;
+
+    @Outlet
+    private JLabel filePathLabel; // The <label> for the file path
+
+    // --- Manually Created Components ---
+    private RSyntaxTextArea editorPane;
+
+
+    public MainFrame() {
+        super("Sierra UI Previewer");
+        this.renderingEngine = new RenderingEngine();
+
+        // 0. Initialize the Recent Files Manager
+        this.recentFilesManager = new RecentFilesManager(MainFrame.class);
+
+        // 1. Load the UI from the declarative .xml file
+        setContentPane(UILoader.load(this, "MainFrame.xml"));
+
+        // 2. Setup the File Chooser
+        this.fileChooser = new JFileChooser();
+        FileNameExtensionFilter xmlFilter = new FileNameExtensionFilter("XML Files (*.xml)", "xml");
+        fileChooser.setFileFilter(xmlFilter);
+        fileChooser.setAcceptAllFileFilterUsed(false);
+
+        // 3. Setup the menu bar
+        setupMenuBar();
+
+        // 4. Manually set up components not supported by the DTD
+        setupCustomEditor();
+
+        // 5. Set layout for previewPanel
+        previewPanel.setLayout(new BorderLayout());
+
+        // Create the JSplitPane with the editor on the left and the preview on the right
+        JSplitPane splitPane = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, editorScrollPane, previewPanel);
+
+        // Set the initial divider location (e.g., 50% of the window width)
+        splitPane.setDividerLocation(0.5);
+
+        // Ensure space is split equally when the frame is resized
+        splitPane.setResizeWeight(0.5);
+
+        // Create a new main panel to hold the JSplitPane (Center) and the StatusBar (South)
+        JPanel mainContentPanel = new JPanel(new BorderLayout());
+        mainContentPanel.add(splitPane, BorderLayout.CENTER);
+        mainContentPanel.add(statusBar, BorderLayout.SOUTH);
+
+        // Set the new panel as the frame's content pane
+        setContentPane(mainContentPanel);
+
+        // 6. Setup Control Subsystem (Debounce Timer)
+        this.debounceTimer = setupDebounceTimer();
+
+        // 7. Wire editor events
+        editorPane.getDocument().addDocumentListener(new DocumentListener() {
+            @Override
+            public void insertUpdate(DocumentEvent e) {
+                debounceTimer.restart();
+                statusBar.setText("Typing...");
+            }
+
+            @Override
+            public void removeUpdate(DocumentEvent e) {
+                debounceTimer.restart();
+                statusBar.setText("Typing...");
+            }
+
+            @Override
+            public void changedUpdate(DocumentEvent e) {
+                // Style changes, not relevant for text
+            }
+        });
+        
+        // 7. load icon
+         URL iconURL = getClass().getResource("/sierra.png");
+        Image icon = new ImageIcon(iconURL).getImage();
+        this.setIconImage(icon);
+
+        // 8. Trigger an initial render
+        triggerRender();
+    }
+
+    // --- Menu Setup ---
+    /**
+     * Creates and sets the application's menu bar.
+     */
+    private void setupMenuBar() {
+
+        // Open
+        openItem.addActionListener(e -> {
+            int result = fileChooser.showOpenDialog(MainFrame.this);
+            if (result == JFileChooser.APPROVE_OPTION) {
+                File selectedFile = fileChooser.getSelectedFile();
+                loadFile(selectedFile);
+            }
+        });
+
+        updateRecentMenu();
+
+        saveItem.addActionListener(e -> saveFile());
+        saveItem.setEnabled(false); // Disabled until a file is successfully loaded
+
+        // Exit
+        exitItem.addActionListener(e -> {
+            System.exit(0); // Exit the application
+        });
+
+        // --- About Menu ---
+        aboutItem.addActionListener(e -> {
+            JOptionPane.showMessageDialog(
+                    this,
+                    """
+                    Sierra DSL Previewer
+                    License: Apache 2.0                    
+                    """,
+                    "About",
+                    JOptionPane.INFORMATION_MESSAGE
+            );
+        });
+
+        this.setJMenuBar(menuBar);
+    }
+
+    /**
+     * Clears and repopulates the Recent Files menu based on the
+     * RecentFilesManager list.
+     */
+    private void updateRecentMenu() {
+        recentMenu.removeAll();
+        List<Path> recentFiles = recentFilesManager.getRecentFiles();
+
+        if (recentFiles.isEmpty()) {
+            recentMenu.setEnabled(false);
+            JMenuItem emptyItem = new JMenuItem("No recent files");
+            emptyItem.setEnabled(false);
+            recentMenu.add(emptyItem);
+        } else {
+            recentMenu.setEnabled(true);
+            for (int i = 0; i < recentFiles.size(); i++) {
+                Path fullPath = recentFiles.get(i);
+
+                // Display only the file name in the menu item, but use the full path for loading
+                String label = (i + 1) + ". " + fullPath.getFileName().toString();
+                JMenuItem item = new JMenuItem(label);
+
+                // Use a local variable for the path in the lambda
+                item.addActionListener(e -> loadFile(fullPath.toFile()));
+                recentMenu.add(item);
+            }
+        }
+    }
+
+    /**
+     * Creates and returns the completion provider for Sierra XML.
+     */
+    private CompletionProvider createCompletionProvider() {
+        return new SierraXMLCompletionProvider();
+    }
+
+    // --- Editor Setup ---
+    /**
+     * Creates the custom RSyntaxTextArea and adds it to the
+     * <scroll-pane> placeholder that Sierra injected.
+     */
+    private void setupCustomEditor() {
+        editorPane = new RSyntaxTextArea(25, 80);
+        editorPane.setSyntaxEditingStyle(SyntaxConstants.SYNTAX_STYLE_XML);
+        editorPane.setCodeFoldingEnabled(true);
+        editorPane.setAntiAliasingEnabled(true);
+        editorPane.setEditable(true); // Ensure it's editable
+
+        CompletionProvider provider = createCompletionProvider();
+
+        // Create the auto-completion manager
+        AutoCompletion ac = new AutoCompletion(provider);
+        ac.setAutoActivationEnabled(true);
+        ac.setAutoActivationDelay(500); // Activate after 500ms of typing
+
+        // Install the auto-completion on the editor pane
+        if (provider != null) {
+            ac.install(editorPane);
+        }
+
+        editorScrollPane.setViewportView(editorPane);
+    }
+
+    // --- Rendering/Control Logic ---
+    /**
+     * Implements the debounce mechanism.
+     */
+    private Timer setupDebounceTimer() {
+        Timer timer = new Timer(1000, (e) -> triggerRender());
+        timer.setRepeats(false);
+        return timer;
+    }
+
+    /**
+     * Kicks off the rendering process on a background thread.
+     */
+    private void triggerRender() {
+        statusBar.setText("Rendering...");
+        String xmlText = editorPane.getText();
+
+        RenderWorker worker = new RenderWorker(xmlText, currentFilePath, renderingEngine, this::displayRenderResult);
+        worker.execute();
+    }
+
+    /**
+     * This is the callback that runs on the EDT when the SwingWorker is done.
+     */
+    private void displayRenderResult(RenderResult result) {
+        switch (result) {
+            case RenderResult.Success success -> {
+                previewPanel.removeAll();
+                JComponent component = success.component();
+                previewPanel.add(component, BorderLayout.CENTER);
+                previewPanel.revalidate();
+                previewPanel.repaint();
+
+                this.setTitle("Sierra UI Previewer");
+                if (saveItem == null || saveItem.isEnabled()) {
+                    statusBar.setText("Render successful.");
+                }
+            }
+            case RenderResult.Error error -> {
+                String errorMessage = error.details().toString();
+                statusBar.setText("Error: " + errorMessage);
+            }
+            default -> {
+            }
+        }
+    }
+
+    // --- FILE LOAD/SAVE LOGIC ---
+    /**
+     * Kicks off a SwingWorker to load a file's content onto a background
+     * thread.
+     */
+    private void loadFile(File file) {
+        // Reset save state while loading
+        saveItem.setEnabled(false);
+        currentFilePath = null;
+
+        filePathLabel.setText("Loading " + file.getName() + "...");
+        FileLoaderWorker worker = new FileLoaderWorker(file.toPath(), this::displayFileContent);
+        worker.execute();
+    }
+
+    /**
+     * Callback that runs on the EDT after the file is loaded. Updates the UI
+     * with the file content and path.
+     */
+    private void displayFileContent(FileLoadResult result) {
+        switch (result) {
+            case FileLoadResult.Success success -> {
+                editorPane.setText(success.content());
+                editorPane.setCaretPosition(0);
+
+                // Set file state and enable Save
+                currentFilePath = success.path();
+                filePathLabel.setText(success.path().toAbsolutePath().toString());
+                saveItem.setEnabled(true);
+
+                // NEW: Update the Recent Files list via the manager
+                recentFilesManager.addFile(currentFilePath);
+                updateRecentMenu(); // Refresh the menu display
+
+                // Re-render the preview with the new content
+                triggerRender();
+            }
+            case FileLoadResult.Error error -> {
+                filePathLabel.setText("Error loading file.");
+                JOptionPane.showMessageDialog(
+                        this,
+                        "Could not read file:\n" + error.exception().getMessage(),
+                        "File Load Error",
+                        JOptionPane.ERROR_MESSAGE
+                );
+            }
+            default -> {
+            }
+        }
+    }
+
+    /**
+     * Kicks off a SwingWorker to save the current content back to the loaded
+     * file path.
+     */
+    private void saveFile() {
+        if (currentFilePath == null) {
+            JOptionPane.showMessageDialog(this, "No file is currently open.", "Save Error", JOptionPane.ERROR_MESSAGE);
+            return;
+        }
+
+        saveItem.setEnabled(false); // Disable save during save operation
+        statusBar.setText("Saving to " + currentFilePath.getFileName() + "...");
+
+        String content = editorPane.getText();
+
+        FileSaverWorker worker = new FileSaverWorker(currentFilePath, content, this::displaySaveResult);
+        worker.execute();
+    }
+
+    /**
+     * Callback that runs on the EDT after the file is saved.
+     */
+    private void displaySaveResult(FileSaveResult result) {
+        saveItem.setEnabled(true); // Re-enable save
+
+        if (result instanceof FileSaveResult.Success) {
+            statusBar.setText("File saved successfully.");
+        } else if (result instanceof FileSaveResult.Error error) {
+            statusBar.setText("Save failed.");
+            JOptionPane.showMessageDialog(
+                    this,
+                    "Could not save file:\n" + error.exception().getMessage(),
+                    "File Save Error",
+                    JOptionPane.ERROR_MESSAGE
+            );
+        }
+    }
+
+    // --- INNER CLASSES FOR FILE LOAD/SAVE ---
+    private sealed interface FileLoadResult {
+
+        record Success(String content, Path path) implements FileLoadResult {
+
+        }
+
+        record Error(Exception exception) implements FileLoadResult {
+
+        }
+    }
+
+    private static class FileLoaderWorker extends SwingWorker<FileLoadResult, Void> {
+
+        private final Path filePath;
+        private final Consumer<FileLoadResult> callback;
+
+        FileLoaderWorker(Path filePath, Consumer<FileLoadResult> callback) {
+            this.filePath = filePath;
+            this.callback = callback;
+        }
+
+        @Override
+        protected FileLoadResult doInBackground() {
+            try {
+                String content = Files.readString(filePath);
+                return new FileLoadResult.Success(content, filePath);
+            } catch (IOException e) {
+                return new FileLoadResult.Error(e);
+            }
+        }
+
+        @Override
+        protected void done() {
+            try {
+                FileLoadResult result = get();
+                callback.accept(result);
+            } catch (InterruptedException | ExecutionException e) {
+                callback.accept(new FileLoadResult.Error(e));
+            }
+        }
+    }
+
+    private sealed interface FileSaveResult {
+
+        record Success() implements FileSaveResult {
+        }
+
+        record Error(Exception exception) implements FileSaveResult {
+
+        }
+    }
+
+    private static class FileSaverWorker extends SwingWorker<FileSaveResult, Void> {
+
+        private final Path filePath;
+        private final String content;
+        private final Consumer<FileSaveResult> callback;
+
+        FileSaverWorker(Path filePath, String content, Consumer<FileSaveResult> callback) {
+            this.filePath = filePath;
+            this.content = content;
+            this.callback = callback;
+        }
+
+        @Override
+        protected FileSaveResult doInBackground() {
+            try {
+                // Write content back to the original file, replacing the old content
+                Files.writeString(filePath, content, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING);
+                return new FileSaveResult.Success();
+            } catch (IOException e) {
+                return new FileSaveResult.Error(e);
+            }
+        }
+
+        @Override
+        protected void done() {
+            try {
+                FileSaveResult result = get();
+                callback.accept(result);
+            } catch (InterruptedException | ExecutionException e) {
+                callback.accept(new FileSaveResult.Error(e));
+            }
+        }
+    }
+
+    // --- INNER CLASS FOR RENDERING ---
+    private static class RenderWorker extends SwingWorker<RenderResult, Void> {
+
+        private final String xmlText;
+        private final Path targetPath; // ADDED: Field to hold the target path
+        private final RenderingEngine engine;
+        private final Consumer<RenderResult> callback;
+
+        // MODIFIED: Constructor now accepts Path
+        RenderWorker(String xmlText, Path targetPath, RenderingEngine engine, Consumer<RenderResult> callback) {
+            this.xmlText = xmlText;
+            this.targetPath = targetPath; // Store the path
+            this.engine = engine;
+            this.callback = callback;
+        }
+
+        @Override
+        protected RenderResult doInBackground() throws Exception {
+            // MODIFIED: Pass the path to the rendering engine
+            return engine.render(xmlText, targetPath);
+        }
+
+        @Override
+        protected void done() {
+            try {
+                RenderResult result = get();
+                callback.accept(result);
+            } catch (InterruptedException | ExecutionException e) {
+                System.out.println(e);
+                callback.accept(new RenderResult.Error(new RenderError(e.getMessage(), e)));
+            }
+        }
+    }
 }
diff --git a/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/RecentFilesManager.java b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/RecentFilesManager.java
new file mode 100644
index 0000000..8443daa
--- /dev/null
+++ b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/RecentFilesManager.java
@@ -0,0 +1,108 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.httprpc.sierra.previewer;
+
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.prefs.Preferences;
+import java.util.stream.Collectors;
+
+/**
+ * Manages the list of recent files using the Preferences API for persistence.
+ */
+public class RecentFilesManager {
+
+    private static final int MAX_RECENT_FILES = 5;
+    private static final String PREF_KEY_RECENT_FILES = "recent_files_list";
+    private static final String PATH_SEPARATOR = ";"; // Used to delimit paths in the stored string
+
+    private final Preferences prefs;
+    private final List<Path> recentFiles = new ArrayList<>();
+
+    public RecentFilesManager(Class<?> applicationClass) {
+        // Get the preferences node for the application
+        this.prefs = Preferences.userNodeForPackage(applicationClass);
+        loadRecentFiles();
+    }
+
+    /**
+     * Loads the recent files list from the Preferences store.
+     */
+    private void loadRecentFiles() {
+        String savedList = prefs.get(PREF_KEY_RECENT_FILES, "");
+
+        if (!savedList.isEmpty()) {
+            // Split the string by the separator, convert to Path objects, and collect.
+            recentFiles.addAll(Arrays.stream(savedList.split(PATH_SEPARATOR))
+                    .filter(s -> !s.trim().isEmpty())
+                    .map(Paths::get)
+                    .collect(Collectors.toList()));
+        }
+    }
+
+    /**
+     * Saves the current recent files list to the Preferences store.
+     */
+    private void saveRecentFiles() {
+        // Convert the list of Paths to a single semicolon-separated string
+        String listToSave = recentFiles.stream()
+                .map(Path::toString)
+                .collect(Collectors.joining(PATH_SEPARATOR));
+
+        prefs.put(PREF_KEY_RECENT_FILES, listToSave);
+    }
+
+    /**
+     * Adds a file path to the recent files list, moving it to the top.
+     * The list is capped at MAX_RECENT_FILES.
+     *
+     * @param path The path of the file to add.
+     */
+    public void addFile(Path path) {
+        // 1. Remove existing entry to ensure it moves to the top
+        recentFiles.remove(path);
+
+        // 2. Add to the front (most recent)
+        recentFiles.add(0, path);
+
+        // 3. Trim the list if it exceeds the max size
+        if (recentFiles.size() > MAX_RECENT_FILES) {
+            recentFiles.remove(MAX_RECENT_FILES);
+        }
+
+        // 4. Save the updated list
+        saveRecentFiles();
+    }
+
+    /**
+     * Returns a copy of the recent files list, sorted by most recent first.
+     *
+     * @return An unmodifiable list of recent file paths.
+     */
+    public List<Path> getRecentFiles() {
+        return recentFiles;
+    }
+
+    /**
+     * Gets the maximum number of files stored.
+     *
+     * @return The maximum size of the list.
+     */
+    public int getMaxFiles() {
+        return MAX_RECENT_FILES;
+    }
+}
\ No newline at end of file
diff --git a/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/SierraPreviewerApp.java b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/SierraPreviewerApp.java
new file mode 100644
index 0000000..57d3f60
--- /dev/null
+++ b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/SierraPreviewerApp.java
@@ -0,0 +1,38 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.httprpc.sierra.previewer;
+
+import com.formdev.flatlaf.FlatLightLaf;
+import javax.swing.*;
+
+public class SierraPreviewerApp {
+
+    public static void main(String[] args) {
+        // Run all UI code on the Event Dispatch Thread (EDT)
+        SwingUtilities.invokeLater(() -> {
+            try {
+                // Set a modern Look and Feel if available
+                UIManager.setLookAndFeel(new FlatLightLaf());
+            } catch (Exception e) {
+                System.err.println("Could not set System Look and Feel.");
+            }
+            
+            MainFrame frame = new MainFrame();
+            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
+            frame.setSize(1200, 800);
+            frame.setLocationRelativeTo(null); // Center on screen
+            frame.setVisible(true);
+        });
+    }
+}
\ No newline at end of file
diff --git a/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/SierraXMLCompletionProvider.java b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/SierraXMLCompletionProvider.java
new file mode 100644
index 0000000..cddeb11
--- /dev/null
+++ b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/SierraXMLCompletionProvider.java
@@ -0,0 +1,302 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.httprpc.sierra.previewer;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import javax.swing.text.BadLocationException;
+import javax.swing.text.JTextComponent;
+
+import org.fife.ui.autocomplete.BasicCompletion;
+import org.fife.ui.autocomplete.Completion;
+import org.fife.ui.autocomplete.DefaultCompletionProvider;
+
+/**
+ * Provides context-aware autocompletion for Sierra DSL XML by dynamically 
+ * parsing the sierra.dtd to obtain valid elements and attributes for the 
+ * current tag. Includes attribute value definitions in the description.
+ */
+public class SierraXMLCompletionProvider extends DefaultCompletionProvider {
+
+    // --- DTD Parsing Regex ---
+    private static final Pattern ELEMENT_PATTERN = Pattern.compile("<!ELEMENT\\s+([a-z-]+)");
+    private static final Pattern ENTITY_PATTERN = Pattern.compile("<!ENTITY\\s+%\\s*([A-Za-z0-9]+)\\s+\"([^\"]+)\"");
+    private static final Pattern ATTLIST_PATTERN = Pattern.compile("<!ATTLIST\\s+([a-z-]+)\\s+%([A-Za-z0-9]+);");
+
+    // Regex to find already defined attributes in a tag, e.g., focusable="true"
+    private static final Pattern DEFINED_ATTR_PATTERN = Pattern.compile("\\s+([a-zA-Z]+)\\s*=");
+
+    // --- Lookup Maps ---
+    private final Map<String, String> entityDefinitions = new HashMap<>();
+    
+    // NEW: Map to store the final resolved attributes for each tag
+    // TagName -> {AttributeName -> Description/ValueDefinitionString}
+    private final Map<String, Map<String, String>> elementAttributeDefinitions = new HashMap<>();
+    
+    private final List<Completion> tagCompletions = new ArrayList<>();
+
+    public SierraXMLCompletionProvider() {
+        String dtdContent = loadDTDContent("/sierra.dtd");
+        parseDTD(dtdContent);
+        setAutoActivationRules(true, "< ");
+    }
+
+    private String loadDTDContent(String path) {
+        StringBuilder content = new StringBuilder();
+        try (InputStream is = getClass().getResourceAsStream(path)) {
+            if (is == null) {
+                System.err.println("Error: Could not find " + path + " in resources.");
+                return "";
+            }
+            InputStreamReader isr = new InputStreamReader(is, StandardCharsets.UTF_8);
+            BufferedReader reader = new BufferedReader(isr);
+            String line;
+            while ((line = reader.readLine()) != null) {
+                content.append(line).append("\n");
+            }
+        } catch (IOException e) {
+            System.err.println("Error reading " + path + ": " + e.getMessage());
+        }
+        return content.toString();
+    }
+
+    private void parseDTD(String content) {
+        Map<String, String> elementToEntityName = new HashMap<>();
+        Map<String, Map<String, String>> entityAttributeMap = new HashMap<>();
+
+        // 1. Find all element tags
+        Matcher elementMatcher = ELEMENT_PATTERN.matcher(content);
+        while (elementMatcher.find()) {
+            String tagName = elementMatcher.group(1);
+            tagCompletions.add(new BasicCompletion(this, tagName, "Sierra UI Element"));
+        }
+        tagCompletions.sort(Comparator.comparing(Completion::getInputText)); 
+
+        // 2. Find all entity definitions
+        Matcher entityMatcher = ENTITY_PATTERN.matcher(content);
+        while (entityMatcher.find()) {
+            entityDefinitions.put(entityMatcher.group(1), entityMatcher.group(2));
+        }
+
+        // 3. Find all ATTLIST mappings
+        Matcher attListMatcher = ATTLIST_PATTERN.matcher(content);
+        while (attListMatcher.find()) {
+            elementToEntityName.put(attListMatcher.group(1), attListMatcher.group(2));
+        }
+
+        // 4. Resolve the full attribute definition for every entity
+        for (String entityName : entityDefinitions.keySet()) {
+            Map<String, String> resolvedDetails = new HashMap<>();
+            resolveEntityAttributes(entityName, resolvedDetails, new HashSet<>());
+            entityAttributeMap.put(entityName, resolvedDetails);
+        }
+
+        // 5. Map Elements to their Resolved Attribute Definitions
+        for (Map.Entry<String, String> entry : elementToEntityName.entrySet()) {
+            String tagName = entry.getKey();
+            String entityName = entry.getValue();
+            
+            Map<String, String> attributes = entityAttributeMap.get(entityName);
+            if (attributes != null) {
+                elementAttributeDefinitions.put(tagName, attributes);
+            }
+        }
+    }
+
+/**
+     * Recursively resolves all attributes and their value definitions for a given entity.
+     * Note: This method populates a Map<String, String> with attributeName -> formattedDescription.
+     */
+    private void resolveEntityAttributes(String entityName, Map<String, String> resolvedDetails, Set<String> visited) {
+        if (!entityDefinitions.containsKey(entityName) || visited.contains(entityName)) {
+            return;
+        }
+        visited.add(entityName);
+        String definition = entityDefinitions.get(entityName);
+
+        // 1. Find and resolve parent entities (e.g., "%JComponent;")
+        Pattern parentEntityPattern = Pattern.compile("%([A-Za-z0-9]+);");
+        Matcher parentMatcher = parentEntityPattern.matcher(definition);
+        while (parentMatcher.find()) {
+            // Merge parent details first (current entity's attributes can override)
+            resolveEntityAttributes(parentMatcher.group(1), resolvedDetails, visited);
+        }
+
+        // 2. Find direct attributes in this entity
+        // Pattern: ([a-zA-Z]+) -> attribute name
+        //          \s+
+        //          ((CDATA|\([^)]+\))) -> attribute definition (capturing CDATA or the enumeration)
+        Pattern attrPattern = Pattern.compile("([a-zA-Z]+)\\s+((CDATA|\\([^)]+\\)))");
+        Matcher attrMatcher = attrPattern.matcher(definition);
+        while (attrMatcher.find()) {
+            String attrName = attrMatcher.group(1);
+            String rawDefinition = attrMatcher.group(2).trim();
+            
+            String formattedDescription;
+            if (rawDefinition.startsWith("CDATA")) {
+                // MODIFIED: Changed from "Type: String (CDATA)" to just "Type: String"
+                formattedDescription = "Type: String"; 
+            } else if (rawDefinition.startsWith("(")) {
+                // Example: (true|false) -> Values: true, false
+                String values = rawDefinition.substring(1, rawDefinition.length() - 1)
+                                             .replace("|", ", ");
+                formattedDescription = "Values: " + values;
+            } else {
+                formattedDescription = "Type: Undefined"; // Should not happen with the regex
+            }
+            
+            resolvedDetails.put(attrName, formattedDescription);
+        }
+    }
+
+    @Override
+    public List<Completion> getCompletions(JTextComponent comp) {
+        int offset = comp.getCaretPosition();
+        CompletionContext context = getCompletionContext(comp, offset);
+
+        if (context.type == CompletionType.TAG_NAME) {
+            return new ArrayList<>(tagCompletions);
+        }
+
+        if (context.type == CompletionType.ATTRIBUTE_NAME && context.tagName != null) {
+            
+            Map<String, String> allAttributes = elementAttributeDefinitions.get(context.tagName);
+            if (allAttributes == null) {
+                return new ArrayList<>();
+            }
+            
+            Set<String> definedAttributes = getAlreadyDefinedAttributes(comp, context.tagStartOffset, offset);
+            List<String> availableAttributeNames = new ArrayList<>();
+            
+            for (String attr : allAttributes.keySet()) {
+                if (!definedAttributes.contains(attr)) {
+                    availableAttributeNames.add(attr);
+                }
+            }
+            
+            Collections.sort(availableAttributeNames);
+
+            List<Completion> suggestions = new ArrayList<>();
+            for (String attr : availableAttributeNames) {
+                String description = allAttributes.get(attr);
+                
+                // Create a richer HTML tooltip using the full description
+                String tooltip = "<html><b>Attribute for &lt;" + context.tagName + "&gt;</b><br>" + description + "</html>";
+                
+                // Use the brief description (e.g., "Values: true, false") for the main popup list
+                suggestions.add(new BasicCompletion(this, attr, description, tooltip));
+            }
+            return suggestions;
+        }
+        
+        return new ArrayList<>();
+    }
+    
+    // --- Helper methods remain the same ---
+
+    private CompletionContext getCompletionContext(JTextComponent comp, int offset) {
+        // ... (implementation remains the same as previous step) ...
+        try {
+            String text = comp.getText(0, offset);
+            int lastOpenAngle = text.lastIndexOf('<');
+            int lastCloseAngle = text.lastIndexOf('>');
+
+            if (lastOpenAngle == -1 || lastCloseAngle > lastOpenAngle) {
+                return new CompletionContext(CompletionType.NONE);
+            }
+
+            int lastQuote = Math.max(text.lastIndexOf('"'), text.lastIndexOf('\''));
+            if (lastQuote > lastOpenAngle) {
+                int quoteCount = 0;
+                char quoteChar = text.charAt(lastQuote);
+                for (int i = lastOpenAngle; i < offset; i++) {
+                    if (text.charAt(i) == quoteChar) {
+                        quoteCount++;
+                    }
+                }
+                if (quoteCount % 2 != 0) {
+                    return new CompletionContext(CompletionType.NONE);
+                }
+            }
+            
+            String tagContent = text.substring(lastOpenAngle + 1);
+            Matcher m = Pattern.compile("^([a-z-]+)").matcher(tagContent);
+            String tagName = null;
+            if (m.find()) {
+                tagName = m.group(1);
+            }
+
+            String textSinceOpen = text.substring(lastOpenAngle);
+            if (!textSinceOpen.contains(" ") && !textSinceOpen.contains("\n")) {
+                return new CompletionContext(CompletionType.TAG_NAME, tagName, lastOpenAngle);
+            }
+
+            char lastChar = text.charAt(offset - 1);
+            if (Character.isWhitespace(lastChar)) {
+                 return new CompletionContext(CompletionType.ATTRIBUTE_NAME, tagName, lastOpenAngle);
+            }
+
+        } catch (BadLocationException e) {
+            // Ignore
+        }
+        return new CompletionContext(CompletionType.NONE);
+    }
+
+    private Set<String> getAlreadyDefinedAttributes(JTextComponent comp, int tagStartOffset, int caretOffset) {
+        Set<String> definedAttributes = new HashSet<>();
+        try {
+            String tagText = comp.getText(tagStartOffset, caretOffset - tagStartOffset);
+            Matcher m = DEFINED_ATTR_PATTERN.matcher(tagText);
+            while (m.find()) {
+                definedAttributes.add(m.group(1));
+            }
+        } catch (BadLocationException e) {
+            // Ignore
+        }
+        return definedAttributes;
+    }
+
+    private enum CompletionType {
+        TAG_NAME, ATTRIBUTE_NAME, NONE
+    }
+
+    private static class CompletionContext {
+        final CompletionType type;
+        final String tagName;
+        final int tagStartOffset;
+
+        CompletionContext(CompletionType type) {
+            this(type, null, -1);
+        }
+        
+        CompletionContext(CompletionType type, String tagName, int tagStartOffset) {
+            this.type = type;
+            this.tagName = tagName;
+            this.tagStartOffset = tagStartOffset;
+        }
+    }
+}
\ No newline at end of file
diff --git a/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/engine/RenderingEngine.java b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/engine/RenderingEngine.java
new file mode 100644
index 0000000..1b68a61
--- /dev/null
+++ b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/engine/RenderingEngine.java
@@ -0,0 +1,69 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.httprpc.sierra.previewer.engine;
+
+import org.httprpc.sierra.previewer.model.RenderError;
+import org.httprpc.sierra.previewer.model.RenderResult;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardOpenOption;
+import javax.swing.*;
+import org.httprpc.sierra.UILoader;
+
+public class RenderingEngine {
+
+    public RenderingEngine() {
+
+    }
+
+    /**
+     * Renders the given XML text by first saving it to a target file.
+     * If targetPath is not null, it saves to that file.
+     * If targetPath is null, it saves to a temporary file.
+     * * @param xmlText The XML content to render.
+     * @param targetPath The file path to save the XML content to. Can be null.
+     * @return The result of the rendering operation.
+     */
+    public RenderResult render(String xmlText, Path targetPath) { // MODIFIED SIGNATURE
+        if (xmlText == null || xmlText.isBlank()) {
+            return new RenderResult.Success(new JPanel());
+        }
+        
+        Path savePath = targetPath;
+
+        try {
+            if (savePath == null) {
+                // If no file is open, create a temporary file as fallback
+                savePath = Files.createTempFile("sierrapreview", ".xml");
+                // Only delete temporary files on exit
+                savePath.toFile().deleteOnExit(); 
+            }
+            
+            // Write the content to the chosen path (either the open file or the temp file)
+            // This is the core change: saving to the targetPath instead of always a temp file.
+            Files.write(savePath, xmlText.getBytes("UTF-8"), 
+                    StandardOpenOption.CREATE, 
+                    StandardOpenOption.TRUNCATE_EXISTING, 
+                    StandardOpenOption.WRITE);
+
+            System.out.println("Data saved for rendering to file: " + savePath.toAbsolutePath());
+
+            JComponent rootComponent = UILoader.load(savePath);
+            return new RenderResult.Success(rootComponent);
+        } catch (IOException e) {
+            return new RenderResult.Error(new RenderError(e.getMessage(), e));
+        }
+    }
+}
\ No newline at end of file
diff --git a/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/model/RenderError.java b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/model/RenderError.java
new file mode 100644
index 0000000..4a8ba5a
--- /dev/null
+++ b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/model/RenderError.java
@@ -0,0 +1,29 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.httprpc.sierra.previewer.model;
+
+/**
+ * A record to hold detailed information about a rendering failure. Implements
+ * the error object described in Section VI.
+ *
+ * @param message A user-friendly error message.
+ * @param exception The underlying exception for debugging.
+ */
+public record RenderError(String message, Exception exception) {
+
+    @Override
+    public String toString() {
+        return message;
+    }
+}
diff --git a/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/model/RenderResult.java b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/model/RenderResult.java
new file mode 100644
index 0000000..b5b9c84
--- /dev/null
+++ b/sierra-previewer/src/main/java/org/httprpc/sierra/previewer/model/RenderResult.java
@@ -0,0 +1,36 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.httprpc.sierra.previewer.model;
+
+import javax.swing.JComponent;
+
+/**
+ * A sealed interface representing the outcome of the rendering process.
+ * It will be either a Success or an Error.
+ */
+public sealed interface RenderResult {
+    /**
+     * Represents a successful render.
+     *
+     * @param component The root JComponent to display.
+     */
+    record Success(JComponent component) implements RenderResult {}
+
+    /**
+     * Represents a failed render.
+     *
+     * @param details The RenderError object.
+     */
+    record Error(RenderError details) implements RenderResult {}
+}
\ No newline at end of file
diff --git a/sierra-previewer/src/main/resources/org/httprpc/sierra/previewer/MainFrame.xml b/sierra-previewer/src/main/resources/org/httprpc/sierra/previewer/MainFrame.xml
index f5c613c..7a4fc00 100644
--- a/sierra-previewer/src/main/resources/org/httprpc/sierra/previewer/MainFrame.xml
+++ b/sierra-previewer/src/main/resources/org/httprpc/sierra/previewer/MainFrame.xml
@@ -1,7 +1,38 @@
-<?xml version="1.0" encoding="utf-8"?>
-
+<?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE column-panel SYSTEM "sierra.dtd">
-
 <column-panel>
-    <!-- TODO -->
-</column-panel>
+    <menu-bar name="menuBar">
+        <menu text="File">
+            <menu-item name="openItem" text="Open..."/>
+            <menu name="recentMenu" text="Recent"/>
+            <popup-menu-separator/>
+            <menu-item name="saveItem" text="Save"/>
+            <popup-menu-separator/>
+            <menu-item name="exitItem" text="Exit"/>
+        </menu>
+        <menu text="About">
+            <menu-item name="aboutItem" text="About Previewer"/>
+        </menu>        
+    </menu-bar>
+
+    
+    <row-panel weight="1">
+
+	<column-panel weight="1">                       
+            <scroll-pane name="editorScrollPane" weight="1"/>            
+        </column-panel>
+
+
+        <column-panel name="previewPanel" padding="5, 5, 5, 5" weight="1">
+            <label text="Preview will appear here."
+                   horizontalAlignment="center"
+                   verticalAlignment="center"
+                   foreground="gray"
+                   weight="1"/>
+        </column-panel>
+
+    </row-panel>
+
+    <label name="filePathLabel" text="" padding="4, 6, 4, 6"/>
+    <label name="statusBar" text="Ready." padding="2, 5, 2, 5"/>
+</column-panel>
\ No newline at end of file
diff --git a/sierra-previewer/src/main/resources/sierra.dtd b/sierra-previewer/src/main/resources/sierra.dtd
new file mode 100644
index 0000000..a3f336b
--- /dev/null
+++ b/sierra-previewer/src/main/resources/sierra.dtd
@@ -0,0 +1,107 @@
+<!ENTITY % UILoader "name CDATA group CDATA border CDATA padding CDATA weight CDATA size CDATA style CDATA styleClass CDATA ">
+<!ENTITY % Component "%UILoader; background CDATA enabled (true|false) focusTraversalKeysEnabled (true|false) focusable (true|false) font CDATA foreground CDATA ignoreRepaint (true|false) name CDATA visible (true|false) ">
+<!ENTITY % Container "%Component; focusCycleRoot (true|false) focusTraversalPolicyProvider (true|false) font CDATA ">
+<!ENTITY % JComponent "%Container; alignmentX CDATA alignmentY CDATA autoscrolls (true|false) background CDATA debugGraphicsOptions CDATA doubleBuffered (true|false) enabled (true|false) font CDATA foreground CDATA inheritsPopupMenu (true|false) opaque (true|false) requestFocusEnabled (true|false) toolTipText CDATA verifyInputWhenFocusTarget (true|false) visible (true|false) ">
+<!ENTITY % AbstractButton "%JComponent; actionCommand CDATA borderPainted (true|false) contentAreaFilled (true|false) disabledIcon CDATA disabledSelectedIcon CDATA displayedMnemonicIndex CDATA enabled (true|false) focusPainted (true|false) hideActionText (true|false) horizontalAlignment (left|center|right|leading|trailing) horizontalTextPosition CDATA icon CDATA iconTextGap CDATA label CDATA mnemonic CDATA multiClickThreshhold CDATA pressedIcon CDATA rolloverEnabled (true|false) rolloverIcon CDATA rolloverSelectedIcon CDATA selected (true|false) selectedIcon CDATA text CDATA verticalAlignment (top|center|bottom) verticalTextPosition CDATA ">
+<!ENTITY % ActivityIndicator "%JComponent; indicatorSize CDATA ">
+<!ELEMENT activity-indicator EMPTY>
+<!ATTLIST activity-indicator %ActivityIndicator;>
+<!ENTITY % ImagePane "%JComponent; horizontalAlignment (leading|trailing|center) image CDATA scaleMode (none|fill-width|fill-height) verticalAlignment (top|bottom|center) ">
+<!ELEMENT image-pane EMPTY>
+<!ATTLIST image-pane %ImagePane;>
+<!ENTITY % JColorChooser "%JComponent; color CDATA dragEnabled (true|false) ">
+<!ELEMENT color-chooser EMPTY>
+<!ATTLIST color-chooser %JColorChooser;>
+<!ENTITY % JComboBox "%JComponent; actionCommand CDATA editable (true|false) enabled (true|false) lightWeightPopupEnabled (true|false) maximumRowCount CDATA popupVisible (true|false) selectedIndex CDATA ">
+<!ELEMENT combo-box EMPTY>
+<!ATTLIST combo-box %JComboBox;>
+<!ENTITY % JLabel "%JComponent; disabledIcon CDATA displayedMnemonic CDATA displayedMnemonicIndex CDATA horizontalAlignment (left|center|right|leading|trailing) horizontalTextPosition CDATA icon CDATA iconTextGap CDATA text CDATA verticalAlignment (top|center|bottom) verticalTextPosition CDATA ">
+<!ELEMENT label EMPTY>
+<!ATTLIST label %JLabel;>
+<!ENTITY % JList "%JComponent; dragEnabled (true|false) dropMode (use-selection|on|insert|insert-rows|insert-cols|on-or-insert|on-or-insert-rows|on-or-insert-cols) fixedCellHeight CDATA fixedCellWidth CDATA layoutOrientation CDATA selectedIndex CDATA selectionBackground CDATA selectionForeground CDATA selectionMode CDATA valueIsAdjusting (true|false) visibleRowCount CDATA ">
+<!ELEMENT list EMPTY>
+<!ATTLIST list %JList;>
+<!ENTITY % JPanel "%JComponent; ">
+<!ENTITY % JProgressBar "%JComponent; borderPainted (true|false) indeterminate (true|false) maximum CDATA minimum CDATA orientation (horizontal|vertical) string CDATA stringPainted (true|false) value CDATA ">
+<!ELEMENT progress-bar EMPTY>
+<!ATTLIST progress-bar %JProgressBar;>
+<!ENTITY % JScrollPane "%JComponent; horizontalScrollBarPolicy CDATA verticalScrollBarPolicy CDATA wheelScrollingEnabled (true|false) ">
+<!ELEMENT scroll-pane (ANY)>
+<!ATTLIST scroll-pane %JScrollPane;>
+<!ENTITY % JSeparator "%JComponent; orientation (horizontal|vertical) ">
+<!ELEMENT separator EMPTY>
+<!ATTLIST separator %JSeparator;>
+<!ENTITY % JSlider "%JComponent; extent CDATA font CDATA inverted (true|false) majorTickSpacing CDATA maximum CDATA minimum CDATA minorTickSpacing CDATA orientation (horizontal|vertical) paintLabels (true|false) paintTicks (true|false) paintTrack (true|false) snapToTicks (true|false) value CDATA valueIsAdjusting (true|false) ">
+<!ELEMENT slider EMPTY>
+<!ATTLIST slider %JSlider;>
+<!ENTITY % JSpinner "%JComponent; ">
+<!ELEMENT spinner EMPTY>
+<!ATTLIST spinner %JSpinner;>
+<!ENTITY % JTable "%JComponent; autoCreateColumnsFromModel (true|false) autoCreateRowSorter (true|false) autoResizeMode CDATA cellSelectionEnabled (true|false) columnSelectionAllowed (true|false) dragEnabled (true|false) dropMode (use-selection|on|insert|insert-rows|insert-cols|on-or-insert|on-or-insert-rows|on-or-insert-cols) editingColumn CDATA editingRow CDATA fillsViewportHeight (true|false) gridColor CDATA rowHeight CDATA rowMargin CDATA rowSelectionAllowed (true|false) selectionBackground CDATA selectionForeground CDATA showHorizontalLines (true|false) showVerticalLines (true|false) surrendersFocusOnKeystroke (true|false) updateSelectionOnSort (true|false) ">
+<!ELEMENT table EMPTY>
+<!ATTLIST table %JTable;>
+<!ENTITY % JTextComponent "%JComponent; caretColor CDATA caretPosition CDATA disabledTextColor CDATA dragEnabled (true|false) dropMode (use-selection|on|insert|insert-rows|insert-cols|on-or-insert|on-or-insert-rows|on-or-insert-cols) editable (true|false) focusAccelerator CDATA selectedTextColor CDATA selectionColor CDATA selectionEnd CDATA selectionStart CDATA text CDATA ">
+<!ENTITY % JTree "%JComponent; dragEnabled (true|false) dropMode (use-selection|on|insert|insert-rows|insert-cols|on-or-insert|on-or-insert-rows|on-or-insert-cols) editable (true|false) expandsSelectedPaths (true|false) invokesStopCellEditing (true|false) largeModel (true|false) rootVisible (true|false) rowHeight CDATA scrollsOnExpand (true|false) showsRootHandles (true|false) toggleClickCount CDATA visibleRowCount CDATA ">
+<!ELEMENT tree EMPTY>
+<!ATTLIST tree %JTree;>
+<!ENTITY % Spacer "%JComponent; ">
+<!ELEMENT spacer EMPTY>
+<!ATTLIST spacer %Spacer;>
+<!ENTITY % TextPane "%JComponent; horizontalAlignment (leading|trailing|center) text CDATA verticalAlignment (top|bottom|center) wrapText (true|false) ">
+<!ELEMENT text-pane EMPTY>
+<!ATTLIST text-pane %TextPane;>
+<!ENTITY % JButton "%AbstractButton; defaultCapable (true|false) ">
+<!ELEMENT button EMPTY>
+<!ATTLIST button %JButton;>
+<!ENTITY % JTextArea "%JTextComponent; columns CDATA font CDATA lineWrap (true|false) rows CDATA tabSize CDATA wrapStyleWord (true|false) ">
+<!ELEMENT text-area EMPTY>
+<!ATTLIST text-area %JTextArea;>
+<!ENTITY % JTextField "%JTextComponent; columns CDATA font CDATA horizontalAlignment (left|center|right|leading|trailing) scrollOffset CDATA placeholderText CDATA showClearButton (true|false) leadingIcon CDATA trailingIcon CDATA ">
+<!ELEMENT text-field EMPTY>
+<!ATTLIST text-field %JTextField;>
+<!ENTITY % JToggleButton "%AbstractButton; ">
+<!ELEMENT toggle-button EMPTY>
+<!ATTLIST toggle-button %JToggleButton;>
+<!ENTITY % LayoutPanel "%JPanel; scrollableTracksViewportHeight (true|false) scrollableTracksViewportWidth (true|false) ">
+<!ENTITY % BoxPanel "%LayoutPanel; spacing CDATA ">
+<!ENTITY % JCheckBox "%JToggleButton; borderPaintedFlat (true|false) ">
+<!ELEMENT check-box EMPTY>
+<!ATTLIST check-box %JCheckBox;>
+<!ENTITY % JFormattedTextField "%JTextField; focusLostBehavior (commit|commit-or-revert|revert|persist) ">
+<!ELEMENT formatted-text-field EMPTY>
+<!ATTLIST formatted-text-field %JFormattedTextField;>
+<!ENTITY % JPasswordField "%JTextField; echoChar CDATA text CDATA ">
+<!ELEMENT password-field EMPTY>
+<!ATTLIST password-field %JPasswordField;>
+<!ENTITY % JRadioButton "%JToggleButton; ">
+<!ELEMENT radio-button EMPTY>
+<!ATTLIST radio-button %JRadioButton;>
+<!ENTITY % MenuButton "%JButton; popupHorizontalAlignment (leading|trailing|center) popupVerticalAlignment (top|bottom|center) ">
+<!ELEMENT menu-button EMPTY>
+<!ATTLIST menu-button %MenuButton;>
+<!ENTITY % NumberField "%JTextField; value CDATA ">
+<!ELEMENT number-field EMPTY>
+<!ATTLIST number-field %NumberField;>
+<!ENTITY % Picker "%JTextField; popupHorizontalAlignment (leading|trailing|center) popupVerticalAlignment (top|bottom|center) ">
+<!ENTITY % StackPanel "%LayoutPanel; ">
+<!ELEMENT stack-panel (ANY)>
+<!ATTLIST stack-panel %StackPanel;>
+<!ENTITY % ValidatedTextField "%JTextField; pattern CDATA value CDATA ">
+<!ELEMENT validated-text-field EMPTY>
+<!ATTLIST validated-text-field %ValidatedTextField;>
+<!ENTITY % ColumnPanel "%BoxPanel; alignToGrid (true|false) ">
+<!ELEMENT column-panel (ANY)>
+<!ATTLIST column-panel %ColumnPanel;>
+<!ENTITY % RowPanel "%BoxPanel; alignToBaseline (true|false) ">
+<!ELEMENT row-panel (ANY)>
+<!ATTLIST row-panel %RowPanel;>
+<!ENTITY % SuggestionPicker "%Picker; maximumRowCount CDATA ">
+<!ELEMENT suggestion-picker EMPTY>
+<!ATTLIST suggestion-picker %SuggestionPicker;>
+<!ENTITY % TemporalPicker "%Picker; ">
+<!ENTITY % DatePicker "%TemporalPicker; ">
+<!ELEMENT date-picker EMPTY>
+<!ATTLIST date-picker %DatePicker;>
+<!ENTITY % TimePicker "%TemporalPicker; minuteInterval CDATA strict (true|false) ">
+<!ELEMENT time-picker EMPTY>
+<!ATTLIST time-picker %TimePicker;>
diff --git a/sierra-previewer/src/main/resources/sierra.png b/sierra-previewer/src/main/resources/sierra.png
new file mode 100644
index 0000000..c595fc0
Binary files /dev/null and b/sierra-previewer/src/main/resources/sierra.png differ
diff --git a/sierra-previewer/src/test/java/org/httprpc/sierra/previewer/SierraXMLCompletionProviderTest.java b/sierra-previewer/src/test/java/org/httprpc/sierra/previewer/SierraXMLCompletionProviderTest.java
new file mode 100644
index 0000000..ce63b2a
--- /dev/null
+++ b/sierra-previewer/src/test/java/org/httprpc/sierra/previewer/SierraXMLCompletionProviderTest.java
@@ -0,0 +1,167 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.httprpc.sierra.previewer;
+
+import org.fife.ui.autocomplete.Completion;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import javax.swing.text.BadLocationException;
+import javax.swing.text.JTextComponent;
+import javax.swing.text.PlainDocument;
+import javax.swing.text.Document;
+
+import java.util.List;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.Mockito.*;
+
+/**
+ * Unit test for SierraXMLCompletionProvider focusing only on core element and attribute completion logic.
+ * This test ensures that the provider correctly reads the 'sierra.dtd' and suggests:
+ * 1. All UI element tags when starting a tag.
+ * 2. Relevant attributes when inside a known tag.
+ */
+public class SierraXMLCompletionProviderTest {
+
+    private SierraXMLCompletionProvider provider;
+    private JTextComponent mockComponent;
+
+    @BeforeEach
+    void setUp() {
+        // Initializes the provider, which attempts to load and parse the DTD from the classpath.
+        provider = new SierraXMLCompletionProvider();
+        mockComponent = mock(JTextComponent.class);
+        
+        // Use a simple PlainDocument for getText() calls to work cleanly
+        Document doc = new PlainDocument();
+        when(mockComponent.getDocument()).thenReturn(doc);
+    }
+
+    /**
+     * Helper to simulate text insertion into the component and set the caret position.
+     * It also mocks the getText(int, int) call to read from the internal document.
+     */
+    private void setComponentText(String text, int caretOffset) throws BadLocationException {
+        Document doc = mockComponent.getDocument();
+        doc.remove(0, doc.getLength());
+        doc.insertString(0, text, null);
+        
+        when(mockComponent.getText(anyInt(), anyInt())).thenAnswer(invocation -> {
+            int start = invocation.getArgument(0);
+            int len = invocation.getArgument(1);
+            return doc.getText(start, len);
+        });
+        when(mockComponent.getCaretPosition()).thenReturn(caretOffset);
+    }
+
+    // --- Core Test Case: 1. Tag Completion ---
+    
+    @Test
+    void testCompletion_StartTag_SuggestsAllUIElements() throws BadLocationException {
+        // Simulate typing '<' to trigger tag completion (caret at offset 1)
+        setComponentText("<", 1); 
+        
+        List<Completion> suggestions = provider.getCompletions(mockComponent);
+        assertFalse(suggestions.isEmpty(), "The provider should suggest elements after typing '<'.");
+
+        // Collect the suggested tag names
+        Set<String> tagNames = suggestions.stream()
+            .map(Completion::getInputText)
+            .collect(Collectors.toSet());
+
+        // Verify the total count of elements parsed from the DTD
+        // This count (32) must match the total number of <!ELEMENT ...> tags in sierra.dtd
+        final int EXPECTED_ELEMENT_COUNT = 32; 
+        assertEquals(EXPECTED_ELEMENT_COUNT, suggestions.size(), 
+            "Should have parsed exactly " + EXPECTED_ELEMENT_COUNT + " elements based on the DTD content.");
+
+        // Verify a selection of known tags from the DTD are present
+        assertTrue(tagNames.contains("button"), "Should contain 'button' tag.");
+        assertTrue(tagNames.contains("label"), "Should contain 'label' tag.");
+        assertTrue(tagNames.contains("activity-indicator"), "Should contain 'activity-indicator' tag.");
+        
+        // Ensure suggestions are alphabetically sorted
+        for (int i = 0; i < suggestions.size() - 1; i++) {
+            String current = suggestions.get(i).getInputText();
+            String next = suggestions.get(i + 1).getInputText();
+            assertTrue(current.compareTo(next) <= 0, "Suggestions should be sorted alphabetically.");
+        }
+    }
+    
+    // --- Core Test Case: 2. Attribute Completion for <button> ---
+
+    @Test
+    void testCompletion_InsideButtonTag_SuggestsCoreAttributes() throws BadLocationException {
+        String text = "<button ";
+        int caretOffset = text.length(); // Caret is right after the space in "<button "
+        
+        setComponentText(text, caretOffset);
+        
+        List<Completion> suggestions = provider.getCompletions(mockComponent);
+        assertFalse(suggestions.isEmpty(), "The provider should suggest attributes inside the '<button ' tag.");
+        
+        // Collect the suggested attribute names (InputText)
+        Set<String> attributeNames = suggestions.stream()
+            .map(Completion::getInputText)
+            .collect(Collectors.toSet());
+
+        assertTrue(attributeNames.contains("name"), "Should suggest the mandatory 'name' attribute.");
+        assertTrue(attributeNames.contains("text"), "Should suggest the mandatory 'text' attribute.");
+
+        // Verify other essential attributes are present (from the DTD)
+        assertTrue(attributeNames.contains("background"), "Should also suggest the 'background' attribute.");
+        
+        // Ensure suggestions are sorted alphabetically
+        for (int i = 0; i < suggestions.size() - 1; i++) {
+            String current = suggestions.get(i).getInputText();
+            String next = suggestions.get(i + 1).getInputText();
+            assertTrue(current.compareTo(next) <= 0, "Suggestions should be sorted alphabetically.");
+        }
+    }
+    
+        @Test
+    void testCompletion_InsideTextAreaTag_SuggestsCoreAttributes() throws BadLocationException {
+        String text = "<text-area ";
+        int caretOffset = text.length();
+        
+        setComponentText(text, caretOffset);
+        
+        List<Completion> suggestions = provider.getCompletions(mockComponent);
+        assertFalse(suggestions.isEmpty(), "The provider should suggest attributes inside the 'text-area ' tag.");
+        
+        // Collect the suggested attribute names (InputText)
+        Set<String> attributeNames = suggestions.stream()
+            .map(Completion::getInputText)
+            .collect(Collectors.toSet());
+
+        assertTrue(attributeNames.contains("name"), "Should suggest the mandatory 'name' attribute.");
+        assertTrue(attributeNames.contains("text"), "Should suggest the mandatory 'text' attribute.");
+        
+        // specific to text area                
+        assertTrue(attributeNames.contains("lineWrap"), "Should suggest the mandatory 'lineWrap' attribute.");
+
+        // Verify other essential attributes are present (from the DTD)
+        assertTrue(attributeNames.contains("background"), "Should also suggest the 'background' attribute.");
+        
+        // Ensure suggestions are sorted alphabetically
+        for (int i = 0; i < suggestions.size() - 1; i++) {
+            String current = suggestions.get(i).getInputText();
+            String next = suggestions.get(i + 1).getInputText();
+            assertTrue(current.compareTo(next) <= 0, "Suggestions should be sorted alphabetically.");
+        }
+    }
+}
\ No newline at end of file
diff --git a/sierra-previewer/src/test/resources/sierra.dtd b/sierra-previewer/src/test/resources/sierra.dtd
new file mode 100644
index 0000000..a3f336b
--- /dev/null
+++ b/sierra-previewer/src/test/resources/sierra.dtd
@@ -0,0 +1,107 @@
+<!ENTITY % UILoader "name CDATA group CDATA border CDATA padding CDATA weight CDATA size CDATA style CDATA styleClass CDATA ">
+<!ENTITY % Component "%UILoader; background CDATA enabled (true|false) focusTraversalKeysEnabled (true|false) focusable (true|false) font CDATA foreground CDATA ignoreRepaint (true|false) name CDATA visible (true|false) ">
+<!ENTITY % Container "%Component; focusCycleRoot (true|false) focusTraversalPolicyProvider (true|false) font CDATA ">
+<!ENTITY % JComponent "%Container; alignmentX CDATA alignmentY CDATA autoscrolls (true|false) background CDATA debugGraphicsOptions CDATA doubleBuffered (true|false) enabled (true|false) font CDATA foreground CDATA inheritsPopupMenu (true|false) opaque (true|false) requestFocusEnabled (true|false) toolTipText CDATA verifyInputWhenFocusTarget (true|false) visible (true|false) ">
+<!ENTITY % AbstractButton "%JComponent; actionCommand CDATA borderPainted (true|false) contentAreaFilled (true|false) disabledIcon CDATA disabledSelectedIcon CDATA displayedMnemonicIndex CDATA enabled (true|false) focusPainted (true|false) hideActionText (true|false) horizontalAlignment (left|center|right|leading|trailing) horizontalTextPosition CDATA icon CDATA iconTextGap CDATA label CDATA mnemonic CDATA multiClickThreshhold CDATA pressedIcon CDATA rolloverEnabled (true|false) rolloverIcon CDATA rolloverSelectedIcon CDATA selected (true|false) selectedIcon CDATA text CDATA verticalAlignment (top|center|bottom) verticalTextPosition CDATA ">
+<!ENTITY % ActivityIndicator "%JComponent; indicatorSize CDATA ">
+<!ELEMENT activity-indicator EMPTY>
+<!ATTLIST activity-indicator %ActivityIndicator;>
+<!ENTITY % ImagePane "%JComponent; horizontalAlignment (leading|trailing|center) image CDATA scaleMode (none|fill-width|fill-height) verticalAlignment (top|bottom|center) ">
+<!ELEMENT image-pane EMPTY>
+<!ATTLIST image-pane %ImagePane;>
+<!ENTITY % JColorChooser "%JComponent; color CDATA dragEnabled (true|false) ">
+<!ELEMENT color-chooser EMPTY>
+<!ATTLIST color-chooser %JColorChooser;>
+<!ENTITY % JComboBox "%JComponent; actionCommand CDATA editable (true|false) enabled (true|false) lightWeightPopupEnabled (true|false) maximumRowCount CDATA popupVisible (true|false) selectedIndex CDATA ">
+<!ELEMENT combo-box EMPTY>
+<!ATTLIST combo-box %JComboBox;>
+<!ENTITY % JLabel "%JComponent; disabledIcon CDATA displayedMnemonic CDATA displayedMnemonicIndex CDATA horizontalAlignment (left|center|right|leading|trailing) horizontalTextPosition CDATA icon CDATA iconTextGap CDATA text CDATA verticalAlignment (top|center|bottom) verticalTextPosition CDATA ">
+<!ELEMENT label EMPTY>
+<!ATTLIST label %JLabel;>
+<!ENTITY % JList "%JComponent; dragEnabled (true|false) dropMode (use-selection|on|insert|insert-rows|insert-cols|on-or-insert|on-or-insert-rows|on-or-insert-cols) fixedCellHeight CDATA fixedCellWidth CDATA layoutOrientation CDATA selectedIndex CDATA selectionBackground CDATA selectionForeground CDATA selectionMode CDATA valueIsAdjusting (true|false) visibleRowCount CDATA ">
+<!ELEMENT list EMPTY>
+<!ATTLIST list %JList;>
+<!ENTITY % JPanel "%JComponent; ">
+<!ENTITY % JProgressBar "%JComponent; borderPainted (true|false) indeterminate (true|false) maximum CDATA minimum CDATA orientation (horizontal|vertical) string CDATA stringPainted (true|false) value CDATA ">
+<!ELEMENT progress-bar EMPTY>
+<!ATTLIST progress-bar %JProgressBar;>
+<!ENTITY % JScrollPane "%JComponent; horizontalScrollBarPolicy CDATA verticalScrollBarPolicy CDATA wheelScrollingEnabled (true|false) ">
+<!ELEMENT scroll-pane (ANY)>
+<!ATTLIST scroll-pane %JScrollPane;>
+<!ENTITY % JSeparator "%JComponent; orientation (horizontal|vertical) ">
+<!ELEMENT separator EMPTY>
+<!ATTLIST separator %JSeparator;>
+<!ENTITY % JSlider "%JComponent; extent CDATA font CDATA inverted (true|false) majorTickSpacing CDATA maximum CDATA minimum CDATA minorTickSpacing CDATA orientation (horizontal|vertical) paintLabels (true|false) paintTicks (true|false) paintTrack (true|false) snapToTicks (true|false) value CDATA valueIsAdjusting (true|false) ">
+<!ELEMENT slider EMPTY>
+<!ATTLIST slider %JSlider;>
+<!ENTITY % JSpinner "%JComponent; ">
+<!ELEMENT spinner EMPTY>
+<!ATTLIST spinner %JSpinner;>
+<!ENTITY % JTable "%JComponent; autoCreateColumnsFromModel (true|false) autoCreateRowSorter (true|false) autoResizeMode CDATA cellSelectionEnabled (true|false) columnSelectionAllowed (true|false) dragEnabled (true|false) dropMode (use-selection|on|insert|insert-rows|insert-cols|on-or-insert|on-or-insert-rows|on-or-insert-cols) editingColumn CDATA editingRow CDATA fillsViewportHeight (true|false) gridColor CDATA rowHeight CDATA rowMargin CDATA rowSelectionAllowed (true|false) selectionBackground CDATA selectionForeground CDATA showHorizontalLines (true|false) showVerticalLines (true|false) surrendersFocusOnKeystroke (true|false) updateSelectionOnSort (true|false) ">
+<!ELEMENT table EMPTY>
+<!ATTLIST table %JTable;>
+<!ENTITY % JTextComponent "%JComponent; caretColor CDATA caretPosition CDATA disabledTextColor CDATA dragEnabled (true|false) dropMode (use-selection|on|insert|insert-rows|insert-cols|on-or-insert|on-or-insert-rows|on-or-insert-cols) editable (true|false) focusAccelerator CDATA selectedTextColor CDATA selectionColor CDATA selectionEnd CDATA selectionStart CDATA text CDATA ">
+<!ENTITY % JTree "%JComponent; dragEnabled (true|false) dropMode (use-selection|on|insert|insert-rows|insert-cols|on-or-insert|on-or-insert-rows|on-or-insert-cols) editable (true|false) expandsSelectedPaths (true|false) invokesStopCellEditing (true|false) largeModel (true|false) rootVisible (true|false) rowHeight CDATA scrollsOnExpand (true|false) showsRootHandles (true|false) toggleClickCount CDATA visibleRowCount CDATA ">
+<!ELEMENT tree EMPTY>
+<!ATTLIST tree %JTree;>
+<!ENTITY % Spacer "%JComponent; ">
+<!ELEMENT spacer EMPTY>
+<!ATTLIST spacer %Spacer;>
+<!ENTITY % TextPane "%JComponent; horizontalAlignment (leading|trailing|center) text CDATA verticalAlignment (top|bottom|center) wrapText (true|false) ">
+<!ELEMENT text-pane EMPTY>
+<!ATTLIST text-pane %TextPane;>
+<!ENTITY % JButton "%AbstractButton; defaultCapable (true|false) ">
+<!ELEMENT button EMPTY>
+<!ATTLIST button %JButton;>
+<!ENTITY % JTextArea "%JTextComponent; columns CDATA font CDATA lineWrap (true|false) rows CDATA tabSize CDATA wrapStyleWord (true|false) ">
+<!ELEMENT text-area EMPTY>
+<!ATTLIST text-area %JTextArea;>
+<!ENTITY % JTextField "%JTextComponent; columns CDATA font CDATA horizontalAlignment (left|center|right|leading|trailing) scrollOffset CDATA placeholderText CDATA showClearButton (true|false) leadingIcon CDATA trailingIcon CDATA ">
+<!ELEMENT text-field EMPTY>
+<!ATTLIST text-field %JTextField;>
+<!ENTITY % JToggleButton "%AbstractButton; ">
+<!ELEMENT toggle-button EMPTY>
+<!ATTLIST toggle-button %JToggleButton;>
+<!ENTITY % LayoutPanel "%JPanel; scrollableTracksViewportHeight (true|false) scrollableTracksViewportWidth (true|false) ">
+<!ENTITY % BoxPanel "%LayoutPanel; spacing CDATA ">
+<!ENTITY % JCheckBox "%JToggleButton; borderPaintedFlat (true|false) ">
+<!ELEMENT check-box EMPTY>
+<!ATTLIST check-box %JCheckBox;>
+<!ENTITY % JFormattedTextField "%JTextField; focusLostBehavior (commit|commit-or-revert|revert|persist) ">
+<!ELEMENT formatted-text-field EMPTY>
+<!ATTLIST formatted-text-field %JFormattedTextField;>
+<!ENTITY % JPasswordField "%JTextField; echoChar CDATA text CDATA ">
+<!ELEMENT password-field EMPTY>
+<!ATTLIST password-field %JPasswordField;>
+<!ENTITY % JRadioButton "%JToggleButton; ">
+<!ELEMENT radio-button EMPTY>
+<!ATTLIST radio-button %JRadioButton;>
+<!ENTITY % MenuButton "%JButton; popupHorizontalAlignment (leading|trailing|center) popupVerticalAlignment (top|bottom|center) ">
+<!ELEMENT menu-button EMPTY>
+<!ATTLIST menu-button %MenuButton;>
+<!ENTITY % NumberField "%JTextField; value CDATA ">
+<!ELEMENT number-field EMPTY>
+<!ATTLIST number-field %NumberField;>
+<!ENTITY % Picker "%JTextField; popupHorizontalAlignment (leading|trailing|center) popupVerticalAlignment (top|bottom|center) ">
+<!ENTITY % StackPanel "%LayoutPanel; ">
+<!ELEMENT stack-panel (ANY)>
+<!ATTLIST stack-panel %StackPanel;>
+<!ENTITY % ValidatedTextField "%JTextField; pattern CDATA value CDATA ">
+<!ELEMENT validated-text-field EMPTY>
+<!ATTLIST validated-text-field %ValidatedTextField;>
+<!ENTITY % ColumnPanel "%BoxPanel; alignToGrid (true|false) ">
+<!ELEMENT column-panel (ANY)>
+<!ATTLIST column-panel %ColumnPanel;>
+<!ENTITY % RowPanel "%BoxPanel; alignToBaseline (true|false) ">
+<!ELEMENT row-panel (ANY)>
+<!ATTLIST row-panel %RowPanel;>
+<!ENTITY % SuggestionPicker "%Picker; maximumRowCount CDATA ">
+<!ELEMENT suggestion-picker EMPTY>
+<!ATTLIST suggestion-picker %SuggestionPicker;>
+<!ENTITY % TemporalPicker "%Picker; ">
+<!ENTITY % DatePicker "%TemporalPicker; ">
+<!ELEMENT date-picker EMPTY>
+<!ATTLIST date-picker %DatePicker;>
+<!ENTITY % TimePicker "%TemporalPicker; minuteInterval CDATA strict (true|false) ">
+<!ELEMENT time-picker EMPTY>
+<!ATTLIST time-picker %TimePicker;>