Allow disabling of nesting

Author: FlorianRappl

src/AngleSharp.Css.Tests/Styling/CssCases.cs

@@ -15,6 +15,7 @@ private static ICssStyleSheet ParseSheet(String text)
             {
                 IsIncludingUnknownDeclarations = true,
                 IsIncludingUnknownRules = true,
+                IsExcludingNesting = true,
                 IsToleratingInvalidSelectors = false,
             });
         }

src/AngleSharp.Css/Extensions/StyleCollectionExtensions.cs

@@ -116,15 +116,15 @@ IEnumerable<Tuple<ICssStyleRule, Priority>> MapPriority(ICssStyleRule rule)
                 if (rule.TryMatch(element, null, out var specificity))
                 {
                     yield return Tuple.Create(rule, specificity);
+                }
 
-                    foreach (var subRule in rule.Rules)
+                foreach (var subRule in rule.Rules)
+                {
+                    if (subRule is ICssStyleRule style)
                     {
-                        if (subRule is ICssStyleRule style)
+                        foreach (var item in MapPriority(style))
                         {
-                            foreach (var item in MapPriority(style))
-                            {
-                                yield return item;
-                            }
+                            yield return item;
                         }
                     }
                 }

src/AngleSharp.Css/Parser/CssBuilder.cs

@@ -524,7 +524,7 @@ public void CreateDeclarationWith(ICssProperties properties, ref CssToken token)
             CollectTrivia(ref token);
             var start = token.Position;
 
-            if (token.IsPotentiallyNested() && properties is ICssStyleDeclaration decl && decl.Parent is CssStyleRule style)
+            if (!_options.IsExcludingNesting && token.IsPotentiallyNested() && properties is ICssStyleDeclaration decl && decl.Parent is CssStyleRule style)
             {
                 var rule = new CssStyleRule(style.Owner);
                 var result = CreateStyle(rule, token);

src/AngleSharp.Css/Parser/CssParserOptions.cs

@@ -36,5 +36,18 @@ public Boolean IsToleratingInvalidSelectors
             get;
             set;
         }
+
+        /// <summary>
+        /// Gets or sets if nesting of style rules should be
+        /// disabled; if disabled the interpretation is closer to
+        /// older browser also accepting declarations such as
+        /// "*x-overflow", which would otherwise be an invalid
+        /// nesting rule.
+        /// </summary>
+        public Boolean IsExcludingNesting
+        {
+            get;
+            set;
+        }
     }
 }