Add test for unbalanced html in a header

This is unexpectedly panicking.

Author: ehuss

tests/testsuite/rendering.rs

@@ -283,3 +283,25 @@ Check that the HTML tags are properly balanced.
         })
         .check_main_file("book/chapter_1.html", str!["<div>x<span>foo</span></div>"]);
 }
+
+// Test for bug with unbalanced HTML handling in the heading.
+#[test]
+fn heading_with_unbalanced_html() {
+    BookTest::init(|_| {})
+        .change_file("src/chapter_1.md", "### Option<T>")
+        .run("build", |cmd| {
+            cmd.expect_failure().expect_stderr(str![[r#"
+ INFO Book building has started
+ INFO Running the html backend
+
+thread 'main' ([..]) panicked at crates/mdbook-html/src/html/tree.rs:[..]
+internal error: expected empty tag stack.
+
+                             path: `chapter_1.md`
+element=Element { name: QualName { prefix: None, ns: Atom('http://www.w3.org/1999/xhtml' type=static), local: Atom('h3' type=inline) }, attrs: {}, self_closing: false, was_raw: false }
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
+"#]]);
+        });
+    // .check_main_file("book/chapter_1.html", str![[""]]);
+}