diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9a1b4e6..9167d24 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ Released on Saturday, September 5 2026
 - Fixed `not <known media type>` is always false (#231)
 - Fixed `calc()` computations in AoT-compiled applications (#236) @sebastienros
 - Fixed usage of `calc()` with unitless scaling (multiplication / division)
+- Fixed case-sensitive matching of `and` / `or` in `@supports` and `from` / `to` in `@keyframes` (#240)
 - Added optional CSSOM compliant color seralization (#229) @lahma
 - Added user-preference media features to the render device (#235) @lahma
 - Added media query list evaluation using `IRenderDevice` (#228) @lahma
diff --git a/src/AngleSharp.Css.Tests/Rules/CssKeyframeRule.cs b/src/AngleSharp.Css.Tests/Rules/CssKeyframeRule.cs
index efcc9a5..9fc71c7 100644
--- a/src/AngleSharp.Css.Tests/Rules/CssKeyframeRule.cs
+++ b/src/AngleSharp.Css.Tests/Rules/CssKeyframeRule.cs
@@ -84,5 +84,38 @@ public void KeyframeRuleWithPercentage_Issue128()
             Assert.AreEqual(3, rule.Key.Stops.Count());
             Assert.AreEqual(0, rule.Style.Length);
         }
+
+        [Test]
+        public void KeyframeRuleWithUppercaseFrom()
+        {
+            var rule = ParseKeyframeRule(@"  FROM {
+    margin-left: 0px;
+  }");
+            Assert.IsNotNull(rule);
+            Assert.AreEqual("0%", rule.KeyText);
+            Assert.AreEqual(1, rule.Key.Stops.Count());
+            Assert.AreEqual(1, rule.Style.Length);
+        }
+
+        [Test]
+        public void KeyframeRuleWithUppercaseTo()
+        {
+            var rule = ParseKeyframeRule(@"  TO {
+    margin-left: 200px;
+  }");
+            Assert.IsNotNull(rule);
+            Assert.AreEqual("100%", rule.KeyText);
+            Assert.AreEqual(1, rule.Key.Stops.Count());
+            Assert.AreEqual(1, rule.Style.Length);
+        }
+
+        [Test]
+        public void KeyframeRuleWithMixedCaseFromAndTo()
+        {
+            var rule = ParseKeyframeRule(@"  From, To { }");
+            Assert.IsNotNull(rule);
+            Assert.AreEqual("0%, 100%", rule.KeyText);
+            Assert.AreEqual(2, rule.Key.Stops.Count());
+        }
     }
 }
diff --git a/src/AngleSharp.Css.Tests/Rules/CssSupports.cs b/src/AngleSharp.Css.Tests/Rules/CssSupports.cs
index 1a51280..c419dd1 100644
--- a/src/AngleSharp.Css.Tests/Rules/CssSupports.cs
+++ b/src/AngleSharp.Css.Tests/Rules/CssSupports.cs
@@ -209,5 +209,57 @@ public void SupportsNegatedDisplayFlexRuleWithDeclarations()
             Assert.AreEqual("not (display: flex)", supports.ConditionText);
             Assert.IsFalse(supports.Condition.Check(device));
         }
+
+        [Test]
+        public void SupportsUppercaseAndKeywordRule()
+        {
+            var source = @"@supports ((background-color: red) AND (color: blue)) { }";
+            var sheet = ParseStyleSheet(source);
+            var device = new DefaultRenderDevice();
+            Assert.AreEqual(1, sheet.Rules.Length);
+            Assert.IsInstanceOf<CssSupportsRule>(sheet.Rules[0]);
+            var supports = sheet.Rules[0] as CssSupportsRule;
+            Assert.AreEqual("((background-color: red) and (color: blue))", supports.ConditionText);
+            Assert.IsTrue(supports.Condition.Check(device));
+        }
+
+        [Test]
+        public void SupportsUppercaseOrKeywordRule()
+        {
+            var source = @"@supports ((background-transparency: half) OR (color: blue)) { }";
+            var sheet = ParseStyleSheet(source);
+            var device = new DefaultRenderDevice();
+            Assert.AreEqual(1, sheet.Rules.Length);
+            Assert.IsInstanceOf<CssSupportsRule>(sheet.Rules[0]);
+            var supports = sheet.Rules[0] as CssSupportsRule;
+            Assert.AreEqual("((background-transparency: half) or (color: blue))", supports.ConditionText);
+            Assert.IsTrue(supports.Condition.Check(device));
+        }
+
+        [Test]
+        public void SupportsMixedCaseAndKeywordChainRule()
+        {
+            var source = @"@supports ((background-color: red) And (color: blue) aND (width: 10px)) { }";
+            var sheet = ParseStyleSheet(source);
+            var device = new DefaultRenderDevice();
+            Assert.AreEqual(1, sheet.Rules.Length);
+            Assert.IsInstanceOf<CssSupportsRule>(sheet.Rules[0]);
+            var supports = sheet.Rules[0] as CssSupportsRule;
+            Assert.AreEqual("((background-color: red) and (color: blue) and (width: 10px))", supports.ConditionText);
+            Assert.IsTrue(supports.Condition.Check(device));
+        }
+
+        [Test]
+        public void SupportsUppercaseAndKeywordKeepsInnerRules()
+        {
+            var source = @"@supports (color: red) AND (display: flex) {
+  body { width: 100%; }
+}";
+            var sheet = ParseStyleSheet(source);
+            Assert.AreEqual(1, sheet.Rules.Length);
+            Assert.IsInstanceOf<CssSupportsRule>(sheet.Rules[0]);
+            var supports = sheet.Rules[0] as CssSupportsRule;
+            Assert.AreEqual(1, supports.Rules.Length);
+        }
     }
 }
diff --git a/src/AngleSharp.Css/Parser/Micro/ConditionParser.cs b/src/AngleSharp.Css/Parser/Micro/ConditionParser.cs
index 9fa66f6..55561c8 100644
--- a/src/AngleSharp.Css/Parser/Micro/ConditionParser.cs
+++ b/src/AngleSharp.Css/Parser/Micro/ConditionParser.cs
@@ -57,8 +57,8 @@ private static IConditionFunction ConjunctionOrDisjunction(this StringSource sou
 
             if (ident != null)
             {
-                var isAnd = ident.Is(CssKeywords.And);
-                var isOr = ident.Is(CssKeywords.Or);
+                var isAnd = ident.Isi(CssKeywords.And);
+                var isOr = ident.Isi(CssKeywords.Or);
 
                 if (isAnd || isOr)
                 {
@@ -141,7 +141,7 @@ private static IEnumerable<IConditionFunction> Scan(this StringSource source, St
                 source.SkipSpacesAndComments();
                 ident = source.ParseIdent();
             }
-            while (ident != null && ident.Is(keyword));
+            while (ident != null && ident.Isi(keyword));
 
             return conditions;
         }
diff --git a/src/AngleSharp.Css/Parser/Micro/KeyframeParser.cs b/src/AngleSharp.Css/Parser/Micro/KeyframeParser.cs
index ca6bc7f..8ea41c0 100644
--- a/src/AngleSharp.Css/Parser/Micro/KeyframeParser.cs
+++ b/src/AngleSharp.Css/Parser/Micro/KeyframeParser.cs
@@ -43,11 +43,11 @@ public static IKeyframeSelector ParseKeyframeSelector(this StringSource source)
 
                     stops.Add(test.Value);
                 }
-                else if (id.Is(CssKeywords.From))
+                else if (id.Isi(CssKeywords.From))
                 {
                     stops.Add(0f);
                 }
-                else if (id.Is(CssKeywords.To))
+                else if (id.Isi(CssKeywords.To))
                 {
                     stops.Add(1f);
                 }