Escape property name

Author: MartinWelsch

src/AngleSharp.Css.Tests/Library/StringRepresentation.cs

@@ -83,5 +83,27 @@ public void ConicGradientNotParsedCorrectly_Issue101()
             var css = styleSheet.ToCss();
             Assert.AreEqual("div { background: conic-gradient(rgba(255, 0, 0, 1), rgba(255, 255, 0, 1), rgba(0, 128, 0, 1)) }", css);
         }
+
+        [Test]
+        public void EscapePropertyNames_CustomProperty_Issue120()
+        {
+            var css = @".class { --\/\%\$\!: value }";
+            var styleSheet = ParseStyleSheet(css);
+
+            var generatedCss = styleSheet.ToCss();
+
+            Assert.AreEqual(css, generatedCss);
+        }
+
+        [Test]
+        public void EscapePropertyNames_UnknownDeclaration_Issue120()
+        {
+            var css = @".class { \/\%\$\!: value }";
+            var styleSheet = ParseStyleSheet(css, new CssParserOptions{ IsIncludingUnknownDeclarations = true });
+
+            var generatedCss = styleSheet.ToCss();
+
+            Assert.AreEqual(css, generatedCss);
+        }
     }
 }

src/AngleSharp.Css/Dom/Internal/CssProperty.cs

@@ -86,7 +86,7 @@ public Boolean IsImportant
 
         #region String Representation
 
-        public void ToCss(TextWriter writer, IStyleFormatter formatter) => writer.Write(formatter.Declaration(Name, Value, IsImportant));
+        public void ToCss(TextWriter writer, IStyleFormatter formatter) => writer.Write(formatter.Declaration(CssUtilities.Escape(Name), Value, IsImportant));
 
         #endregion
     }