Add an opt-in switch for CSSOM-compliant color serialization

docs/tutorials/04-Questions.md

@@ -6,24 +6,36 @@ section: "AngleSharp.Css"
 
 ## How to change the color output?
 
-By default, AngleSharp.Css uses `rgba()` for the serialization of `Color`. To change this you can set
+By default, AngleSharp.Css uses `rgba()` for the serialization of `CssColorValue`. To change this you can set
 
 ```cs
-Color.UseHex = true;
+CssColorValue.UseHex = true;
 ```
 
 which will automatically use hex for all non-transparent colors. All other colors would still be represented via the `rgba()` function.
 
 So you'd get:
 
 ```cs
-Color.UseHex = true;
-var color1 = new Color(65, 12, 48);
+CssColorValue.UseHex = true;
+var color1 = new CssColorValue(65, 12, 48);
 // color1.CssText = #410C30
-var color2 = new Color(65, 12, 48, 10);
+var color2 = new CssColorValue(65, 12, 48, 10);
 // color2.CssText = rgba(65, 12, 48, 0.04)
 ```
 
+Alternatively, you can follow the serialization rules from the CSSOM specification, which omit the alpha channel of an opaque color:
+
+```cs
+CssColorValue.UseSpecSerialization = true;
+var color1 = new CssColorValue(65, 12, 48);
+// color1.CssText = rgb(65, 12, 48)
+var color2 = new CssColorValue(65, 12, 48, 10);
+// color2.CssText = rgba(65, 12, 48, 0.04)
+```
+
+Both switches are global and `UseHex` wins if both are active.
+
 ## Why is my linked stylesheet not loaded?
 
 Most commonly, resource loading is not enabled. For external stylesheets, configure a requester and enable resource loading.

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

@@ -16,6 +16,13 @@ namespace AngleSharp.Css.Tests.Library
     [TestFixture]
     public class StringRepresentationTests
     {
+        [TearDown]
+        public void ResetColorSerialization()
+        {
+            CssColorValue.UseHex = false;
+            CssColorValue.UseSpecSerialization = false;
+        }
+
         [Test]
         public void PrettyStyleFormatterStringifyShouldWork_Issue41()
         {
@@ -50,6 +57,62 @@ public void TransparentColorWorksWithHexOutput_Issue132()
             Assert.AreEqual("#410C300A", text);
         }
 
+        [Test]
+        public void OpaqueColorKeepsTheAlphaChannelByDefault_Issue227()
+        {
+            var color = new CssColorValue(65, 12, 48);
+            Assert.AreEqual("rgba(65, 12, 48, 1)", color.CssText);
+        }
+
+        [Test]
+        public void OpaqueColorDropsTheAlphaChannelWithSpecOutput_Issue227()
+        {
+            var color = new CssColorValue(65, 12, 48);
+            CssColorValue.UseSpecSerialization = true;
+            Assert.AreEqual("rgb(65, 12, 48)", color.CssText);
+        }
+
+        [Test]
+        public void TransparentColorKeepsTheAlphaChannelWithSpecOutput_Issue227()
+        {
+            var color = new CssColorValue(65, 12, 48, 128);
+            CssColorValue.UseSpecSerialization = true;
+            Assert.AreEqual("rgba(65, 12, 48, 0.5)", color.CssText);
+        }
+
+        [Test]
+        public void OpaqueColorPrefersHexOutputOverSpecOutput_Issue227()
+        {
+            var color = new CssColorValue(65, 12, 48);
+            CssColorValue.UseHex = true;
+            CssColorValue.UseSpecSerialization = true;
+            Assert.AreEqual("#410C30", color.CssText);
+        }
+
+        [Test]
+        public void TransparentColorPrefersHexOutputOverSpecOutput_Issue227()
+        {
+            var color = new CssColorValue(65, 12, 48, 10);
+            CssColorValue.UseHex = true;
+            CssColorValue.UseSpecSerialization = true;
+            Assert.AreEqual("#410C300A", color.CssText);
+        }
+
+        [Test]
+        public void CurrentColorIsNotAffectedBySpecOutput_Issue227()
+        {
+            CssColorValue.UseSpecSerialization = true;
+            Assert.AreEqual("currentColor", CssColorValue.CurrentColor.CssText);
+        }
+
+        [Test]
+        public void DeclarationUsesSpecOutputForOpaqueColors_Issue227()
+        {
+            CssColorValue.UseSpecSerialization = true;
+            var declaration = ParseDeclaration("color: rgba(255, 0, 0, 1)");
+            Assert.AreEqual("color: rgb(255, 0, 0)", declaration.CssText);
+        }
+
         [Test]
         public void ShorthandPaddingInheritPropertiesShouldBeIncluded_Issue100()
         {

src/AngleSharp.Css/Values/Primitives/CssColorValue.cs

@@ -469,6 +469,12 @@ public static CssColorValue FromHwba(Double h, Double w, Double b, Double alpha)
         /// </summary>
         public static Boolean UseHex { get; set; }
 
+        /// <summary>
+        /// Gets or sets if the CSSOM serialization rules should be used, i.e.,
+        /// if the alpha channel of an opaque color should be omitted.
+        /// </summary>
+        public static Boolean UseSpecSerialization { get; set; }
+
         /// <summary>
         /// Gets the CSS text representation.
         /// </summary>
@@ -495,6 +501,17 @@ public String CssText
 
                     return color;
                 }
+                else if (UseSpecSerialization && _alpha == 255)
+                {
+                    var fn = FunctionNames.Rgb;
+                    var args = String.Join(", ", new[]
+                    {
+                        R.ToString(CultureInfo.InvariantCulture),
+                        G.ToString(CultureInfo.InvariantCulture),
+                        B.ToString(CultureInfo.InvariantCulture),
+                    });
+                    return fn.CssFunction(args);
+                }
                 else
                 {
                     var fn = FunctionNames.Rgba;