Respect unknown decl rule

src/AngleSharp.Css/BrowsingContextExtensions.cs

@@ -64,7 +64,13 @@ namespace AngleSharp.Css
             var factory = context.GetFactory<IDeclarationFactory>();
             var info = factory.Create(name);
             var value = info.Combine(factory, longhands);
-            return new CssProperty(name, info.Converter, info.Flags, value, important);
+
+            if (context.AllowsDeclaration(info))
+            {
+                return new CssProperty(name, info.Converter, info.Flags, value, important);
+            }
+
+            return null;
         }
 
         internal static ICssProperty[] CreateLonghands(this IBrowsingContext context, ICssProperty shorthand)
@@ -78,9 +84,8 @@ namespace AngleSharp.Css
         internal static CssProperty CreateProperty(this IBrowsingContext context, String propertyName)
         {
             var info = context.GetDeclarationInfo(propertyName);
-            var provider = context.GetProvider<CssParser>();
 
-            if (info.Flags != PropertyFlags.Unknown || context.IsAllowingUnknownDeclarations())
+            if (context.AllowsDeclaration(info))
             {
                 return new CssProperty(propertyName, info.Converter, info.Flags);
             }
@@ -88,6 +93,9 @@ namespace AngleSharp.Css
             return null;
         }
 
+        private static Boolean AllowsDeclaration(this IBrowsingContext context, DeclarationInfo info) =>
+            info.Flags != PropertyFlags.Unknown || context.IsAllowingUnknownDeclarations();
+
         private static Boolean IsAllowingUnknownDeclarations(this IBrowsingContext context)
         {
             var parser = context.GetProvider<CssParser>();

src/AngleSharp.Css/DefaultDeclarationFactory.cs

@@ -1680,10 +1680,7 @@ namespace AngleSharp.Css
         /// </summary>
         /// <param name="propertyName">The name of the property.</param>
         /// <param name="converter">The converter to use.</param>
-        public void Register(String propertyName, DeclarationInfo converter)
-        {
-            _declarations.Add(propertyName, converter);
-        }
+        public void Register(String propertyName, DeclarationInfo converter) => _declarations.Add(propertyName, converter);
 
         /// <summary>
         /// Unregisters an existing declaration.
@@ -1692,9 +1689,7 @@ namespace AngleSharp.Css
         /// <returns>The registered declaration, if any.</returns>
         public DeclarationInfo Unregister(String propertyName)
         {
-            var info = default(DeclarationInfo);
-
-            if (_declarations.TryGetValue(propertyName, out info))
+            if (_declarations.TryGetValue(propertyName, out DeclarationInfo info))
             {
                 _declarations.Remove(propertyName);
             }