Allow empty value for style property

- conforms to CSS specs - use default value if value is empty
- fixes #318

Source/External/ExCSS/Parser.Blocks.cs

@@ -342,8 +342,10 @@ namespace Svg.ExCSS
             switch (token.GrammarSegment)
             {
                 case GrammarSegment.Semicolon:
+                    // the value is empty - remove the property to use the default value
+                    RemoveCurrentProperty();
                     SetParsingContext(ParsingContext.InDeclaration);
-                    break;
+                    return true;
 
                 case GrammarSegment.CurlyBracketClose:
                     ParseDeclaration(token);

Source/External/ExCSS/Parser.cs

@@ -223,6 +223,20 @@ namespace Svg.ExCSS
             }
         }
 
+        private void RemoveCurrentProperty()
+        {
+            if (_property != null)
+            {
+                var rule = CurrentRule as ISupportsDeclarations;
+
+                if (rule != null)
+                {
+                    rule.Declarations.Remove(_property);
+                }
+                _property = null;
+            }
+        }
+
         private T CastRuleSet<T>() where T : RuleSet
         {
             if (_activeRuleSets.Count > 0)

Tests/Svg.UnitTests/PassingTests.csv

@@ -188,6 +188,7 @@ __issue-227-02
 __issue-239-01
 __issue-242-01
 __issue-279-01
+__issue-318-01
 __issue-323-02
 __issue-323-03
 __issue-329-01