Added more tests for background-size

Author: FlorianRappl

src/AngleSharp.Css.Tests/Declarations/CssBackgroundProperty.cs

@@ -6,6 +6,30 @@ namespace AngleSharp.Css.Tests.Declarations
     [TestFixture]
     public class CssBackgroundPropertyTests
     {
+        [Test]
+        public void CssBackgroundSizeCoverTest()
+        {
+            var snippet = "background-size : cover";
+            var property = ParseDeclaration(snippet);
+            Assert.AreEqual("background-size", property.Name);
+            Assert.IsFalse(property.IsImportant);
+            Assert.IsFalse(property.IsInherited);
+            Assert.IsTrue(property.HasValue);
+            Assert.AreEqual("cover", property.Value);
+        }
+
+        [Test]
+        public void CssBackgroundSizeContainTest()
+        {
+            var snippet = "background-size : contain";
+            var property = ParseDeclaration(snippet);
+            Assert.AreEqual("background-size", property.Name);
+            Assert.IsFalse(property.IsImportant);
+            Assert.IsFalse(property.IsInherited);
+            Assert.IsTrue(property.HasValue);
+            Assert.AreEqual("contain", property.Value);
+        }
+
         [Test]
         public void CssBackgroundAttachmentScrollLegal()
         {

src/AngleSharp.Css/Parser/Micro/PointParser.cs

@@ -167,15 +167,15 @@ public static CssBackgroundSizeValue ParseSize(this StringSource source)
             {
                 var w = source.ParseDistanceOrCalc();
 
-                if (w == null && !source.IsIdentifier(CssKeywords.Auto))
+                if (w is null && !source.IsIdentifier(CssKeywords.Auto))
                 {
                     return null;
                 }
 
                 source.SkipSpacesAndComments();
                 var h = source.ParseDistanceOrCalc();
 
-                if (h == null)
+                if (h is null)
                 {
                     source.IsIdentifier(CssKeywords.Auto);
                 }

src/AngleSharp.Css/Values/Composites/CssBackgroundSizeValue.cs

@@ -30,8 +30,8 @@ public sealed class CssBackgroundSizeValue : IEquatable<CssBackgroundSizeValue>,
 
         private CssBackgroundSizeValue(ValueMode mode)
         {
-            _width = Length.Zero;
-            _height = Length.Zero;
+            _width = Length.Auto;
+            _height = Length.Auto;
             _mode = mode;
         }
 
@@ -94,18 +94,29 @@ public String CssText
         /// </summary>
         /// <param name="other">The other background size.</param>
         /// <returns>True if both are equivalent, otherwise false.</returns>
-        public Boolean Equals(CssBackgroundSizeValue other) =>
-            _mode == other._mode && _height == other._height && _width == other._width;
+        public Boolean Equals(CssBackgroundSizeValue other) => _mode == other._mode && _height == other._height && _width == other._width;
 
         #endregion
 
         #region Value Mode
 
+        /// <summary>
+        /// Represents the value modes for the vertical and horizontal axis.
+        /// </summary>
         private enum ValueMode
         {
+            /// <summary>
+            /// The size is explicitly specified.
+            /// </summary>
             Explicit,
+            /// <summary>
+            /// The size should cover the axis.
+            /// </summary>
             Cover,
-            Contain
+            /// <summary>
+            /// The size should contain the axis.
+            /// </summary>
+            Contain,
         }
 
         #endregion