Fix !important declarations being overridden by non-important duplicates

In SetLonghand, a later non-important declaration for the same property
would unconditionally replace an earlier !important one. Now the existing
declaration is preserved when it is !important and the incoming one is not.

Fixes the issue where parsing style="padding: 20px !important; padding: 0"
would incorrectly yield "padding: 0" instead of keeping the important value.

Author: jafin

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

@@ -1,5 +1,7 @@
 namespace AngleSharp.Css.Tests.Declarations
 {
+    using AngleSharp.Css.Dom;
+    using AngleSharp.Dom;
     using NUnit.Framework;
     using static CssConstructionFunctions;
 
@@ -139,5 +141,62 @@ public void CssPaddingShouldBeRecombinedCorrectly()
             var actual = result.CssText;
             Assert.AreEqual(expected, actual);
         }
+
+        [Test]
+        public void CssPaddingImportantShouldNotBeOverriddenByNonImportant()
+        {
+            var snippet = "padding: 20px !important; font-size: 20px; padding: 0";
+            var style = ParseDeclarations(snippet);
+            var padding = style.GetPropertyValue("padding");
+            Assert.AreEqual("20px", padding);
+            var paddingProp = style.GetProperty("padding");
+            Assert.IsNotNull(paddingProp);
+            Assert.IsTrue(paddingProp.IsImportant);
+            var fontSize = style.GetPropertyValue("font-size");
+            Assert.AreEqual("20px", fontSize);
+        }
+
+        [Test]
+        public void CssPaddingImportantShouldBeOverriddenByImportant()
+        {
+            var snippet = "padding: 20px !important; padding: 0 !important";
+            var style = ParseDeclarations(snippet);
+            var padding = style.GetPropertyValue("padding");
+            Assert.AreEqual("0", padding);
+            var paddingProp = style.GetProperty("padding");
+            Assert.IsNotNull(paddingProp);
+            Assert.IsTrue(paddingProp.IsImportant);
+        }
+
+        [Test]
+        public void CssPaddingNonImportantShouldBeOverriddenByNonImportant()
+        {
+            var snippet = "padding: 20px; padding: 0";
+            var style = ParseDeclarations(snippet);
+            var padding = style.GetPropertyValue("padding");
+            Assert.AreEqual("0", padding);
+        }
+
+        [Test]
+        public void CssPaddingNonImportantShouldBeOverriddenByImportant()
+        {
+            var snippet = "padding: 20px; padding: 0 !important";
+            var style = ParseDeclarations(snippet);
+            var padding = style.GetPropertyValue("padding");
+            Assert.AreEqual("0", padding);
+            var paddingProp = style.GetProperty("padding");
+            Assert.IsNotNull(paddingProp);
+            Assert.IsTrue(paddingProp.IsImportant);
+        }
+
+        [Test]
+        public void CssInlineStyleImportantPaddingShouldTakePrecedence()
+        {
+            var html = "<p style=\"padding: 20px !important; font-size: 20px; padding: 0;\">Test</p>";
+            var dom = ParseDocument(html);
+            var p = dom.QuerySelector("p");
+            var style = p.GetStyle();
+            Assert.AreEqual("padding: 20px !important; font-size: 20px", style.CssText);
+        }
     }
 }

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

@@ -424,6 +424,11 @@ private void SetLonghand(ICssProperty property)
 
                 if (declaration.Name.Is(property.Name))
                 {
+                    if (declaration.IsImportant && !property.IsImportant)
+                    {
+                        return;
+                    }
+
                     _declarations[i] = property;
                     return;
                 }