diff --git a/Userland/Libraries/LibWeb/CMakeLists.txt b/Userland/Libraries/LibWeb/CMakeLists.txt
index d7cce14091..9a6f035b4a 100644
--- a/Userland/Libraries/LibWeb/CMakeLists.txt
+++ b/Userland/Libraries/LibWeb/CMakeLists.txt
@@ -47,6 +47,7 @@ set(SOURCES
     CSS/StyleSheet.cpp
     CSS/StyleSheetList.cpp
     CSS/StyleValue.cpp
+    CSS/Supports.cpp
     CSS/ValueID.cpp
     CSS/ValueID.h
     Cookie/ParsedCookie.cpp
diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
index cebf6816e3..a1177b8585 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
@@ -1604,7 +1604,7 @@ RefPtr<PropertyOwningCSSStyleDeclaration> Parser::convert_to_declaration(Nonnull
     return parse_a_list_of_declarations(stream);
 }
 
-Optional<StyleProperty> Parser::convert_to_style_property(StyleDeclarationRule& declaration)
+Optional<StyleProperty> Parser::convert_to_style_property(StyleDeclarationRule const& declaration)
 {
     auto& property_name = declaration.m_name;
     auto property_id = property_id_from_string(property_name);
diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.h b/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
index d3b55de014..e52575f8a4 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
@@ -106,6 +106,9 @@ public:
 
     RefPtr<StyleValue> parse_as_css_value(PropertyID);
 
+    // FIXME: This is a hack, while CSS::Supports is using a StyleDeclarationRule
+    [[nodiscard]] Optional<StyleProperty> convert_to_style_property(StyleDeclarationRule const&);
+
 private:
     enum class ParsingResult {
         Done,
@@ -176,7 +179,6 @@ private:
 
     [[nodiscard]] RefPtr<CSSRule> convert_to_rule(NonnullRefPtr<StyleRule>);
     [[nodiscard]] RefPtr<PropertyOwningCSSStyleDeclaration> convert_to_declaration(NonnullRefPtr<StyleBlockRule>);
-    [[nodiscard]] Optional<StyleProperty> convert_to_style_property(StyleDeclarationRule&);
 
     static Optional<float> try_parse_float(StringView string);
     static Optional<Color> parse_color(ParsingContext const&, StyleComponentValueRule const&);
diff --git a/Userland/Libraries/LibWeb/CSS/Supports.cpp b/Userland/Libraries/LibWeb/CSS/Supports.cpp
new file mode 100644
index 0000000000..bb96088e10
--- /dev/null
+++ b/Userland/Libraries/LibWeb/CSS/Supports.cpp
@@ -0,0 +1,79 @@
+/*
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/CSS/Parser/Parser.h>
+#include <LibWeb/CSS/Supports.h>
+
+namespace Web::CSS {
+
+Supports::Supports(NonnullOwnPtr<Condition>&& condition)
+    : m_condition(move(condition))
+{
+    auto result = m_condition->evaluate();
+    if (result == MatchResult::Unknown) {
+        dbgln("!!! Evaluation of CSS Supports returned 'Unknown'!");
+    }
+    m_matches = result == MatchResult::True;
+}
+
+Supports::MatchResult Supports::Condition::evaluate() const
+{
+    switch (type) {
+    case Type::Not:
+        return negate(children.first().evaluate());
+    case Type::And: {
+        size_t true_results = 0;
+        for (auto& child : children) {
+            auto child_match = child.evaluate();
+            if (child_match == MatchResult::False)
+                return MatchResult::False;
+            if (child_match == MatchResult::True)
+                true_results++;
+        }
+        if (true_results == children.size())
+            return MatchResult::True;
+        return MatchResult::Unknown;
+    }
+    case Type::Or: {
+        size_t false_results = 0;
+        for (auto& child : children) {
+            auto child_match = child.evaluate();
+            if (child_match == MatchResult::True)
+                return MatchResult::True;
+            if (child_match == MatchResult::False)
+                false_results++;
+        }
+        if (false_results == children.size())
+            return MatchResult::False;
+        return MatchResult::Unknown;
+    }
+    }
+    VERIFY_NOT_REACHED();
+}
+
+Supports::MatchResult Supports::InParens::evaluate() const
+{
+    return value.visit(
+        [&](NonnullOwnPtr<Condition>& condition) {
+            return condition->evaluate();
+        },
+        [&](Feature& feature) {
+            return feature.evaluate();
+        },
+        [&](GeneralEnclosed&) {
+            return MatchResult::Unknown;
+        });
+}
+
+Supports::MatchResult Supports::Feature::evaluate() const
+{
+    auto style_property = Parser({}, "").convert_to_style_property(declaration);
+    if (style_property.has_value())
+        return MatchResult::True;
+    return MatchResult::False;
+}
+
+}
diff --git a/Userland/Libraries/LibWeb/CSS/Supports.h b/Userland/Libraries/LibWeb/CSS/Supports.h
new file mode 100644
index 0000000000..6fe96351ce
--- /dev/null
+++ b/Userland/Libraries/LibWeb/CSS/Supports.h
@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullOwnPtr.h>
+#include <AK/RefCounted.h>
+#include <AK/String.h>
+#include <AK/Variant.h>
+#include <AK/Vector.h>
+#include <LibWeb/CSS/Parser/StyleDeclarationRule.h>
+
+namespace Web::CSS {
+
+class Supports final : public RefCounted<Supports> {
+    friend class Parser;
+
+private:
+    enum class MatchResult {
+        False,
+        True,
+        Unknown,
+    };
+
+    static MatchResult negate(MatchResult value)
+    {
+        switch (value) {
+        case MatchResult::False:
+            return MatchResult::True;
+        case MatchResult::True:
+            return MatchResult::False;
+        case MatchResult::Unknown:
+            return MatchResult::Unknown;
+        }
+        VERIFY_NOT_REACHED();
+    }
+
+public:
+    struct GeneralEnclosed {
+    };
+
+    struct Feature {
+        // FIXME: Using this internal parser class is a bit of a hack.
+        StyleDeclarationRule declaration;
+        MatchResult evaluate() const;
+    };
+
+    struct Condition;
+    struct InParens {
+        Variant<NonnullOwnPtr<Condition>, Feature, GeneralEnclosed> value;
+
+        MatchResult evaluate() const;
+    };
+
+    struct Condition {
+        enum class Type {
+            Not,
+            And,
+            Or,
+        };
+        Type type;
+        Vector<InParens> children;
+
+        MatchResult evaluate() const;
+    };
+
+    static NonnullRefPtr<Supports> create(NonnullOwnPtr<Condition>&& condition)
+    {
+        return adopt_ref(*new Supports(move(condition)));
+    }
+
+    bool matches() const { return m_matches; }
+
+private:
+    Supports(NonnullOwnPtr<Condition>&&);
+
+    NonnullOwnPtr<Condition> m_condition;
+    bool m_matches { false };
+};
+
+}
diff --git a/Userland/Libraries/LibWeb/Forward.h b/Userland/Libraries/LibWeb/Forward.h
index c6f3506a80..cdc6d6a33b 100644
--- a/Userland/Libraries/LibWeb/Forward.h
+++ b/Userland/Libraries/LibWeb/Forward.h
@@ -39,6 +39,7 @@ class Selector;
 class StyleProperties;
 class StyleComputer;
 class StyleSheet;
+class Supports;
 
 class StyleValue;
 class BackgroundRepeatStyleValue;