diff --git a/Userland/Libraries/LibWeb/CSS/MediaList.cpp b/Userland/Libraries/LibWeb/CSS/MediaList.cpp
index aa8097f256..b723800519 100644
--- a/Userland/Libraries/LibWeb/CSS/MediaList.cpp
+++ b/Userland/Libraries/LibWeb/CSS/MediaList.cpp
@@ -67,4 +67,21 @@ void MediaList::delete_medium(String medium)
     // FIXME: If nothing was removed, then throw a NotFoundError exception.
 }
 
+bool MediaList::evaluate(DOM::Window const& window)
+{
+    for (auto& media : m_media)
+        media.evaluate(window);
+
+    return matches();
+}
+
+bool MediaList::matches() const
+{
+    for (auto& media : m_media) {
+        if (media.matches())
+            return true;
+    }
+    return false;
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/CSS/MediaList.h b/Userland/Libraries/LibWeb/CSS/MediaList.h
index 4586f731bb..d264654316 100644
--- a/Userland/Libraries/LibWeb/CSS/MediaList.h
+++ b/Userland/Libraries/LibWeb/CSS/MediaList.h
@@ -29,6 +29,9 @@ public:
     void append_medium(String);
     void delete_medium(String);
 
+    bool evaluate(DOM::Window const&);
+    bool matches() const;
+
 private:
     explicit MediaList(NonnullRefPtrVector<MediaQuery>&&);
 
diff --git a/Userland/Libraries/LibWeb/CSS/MediaQuery.cpp b/Userland/Libraries/LibWeb/CSS/MediaQuery.cpp
index 507d28a48a..83effe782f 100644
--- a/Userland/Libraries/LibWeb/CSS/MediaQuery.cpp
+++ b/Userland/Libraries/LibWeb/CSS/MediaQuery.cpp
@@ -5,6 +5,8 @@
  */
 
 #include <LibWeb/CSS/MediaQuery.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/Window.h>
 
 namespace Web::CSS {
 
@@ -33,6 +35,59 @@ String MediaQuery::MediaFeature::to_string() const
     VERIFY_NOT_REACHED();
 }
 
+bool MediaQuery::MediaFeature::evaluate(DOM::Window const& window) const
+{
+    auto queried_value = window.query_media_feature(name);
+    if (!queried_value)
+        return false;
+
+    switch (type) {
+    case Type::IsTrue:
+        if (queried_value->has_number())
+            return queried_value->to_number() != 0;
+        if (queried_value->has_length())
+            return queried_value->to_length().raw_value() != 0;
+        if (queried_value->has_identifier())
+            return queried_value->to_identifier() != ValueID::None;
+        return false;
+
+    case Type::ExactValue:
+        return queried_value->equals(*value);
+
+    case Type::MinValue:
+        if (queried_value->has_number() && value->has_number())
+            return queried_value->to_number() >= value->to_number();
+        if (queried_value->has_length() && value->has_length()) {
+            auto queried_length = queried_value->to_length();
+            auto value_length = value->to_length();
+            // FIXME: We should be checking that lengths are valid during parsing
+            if (!value_length.is_absolute()) {
+                dbgln("Media feature was given a non-absolute length, which is invalid! {}", value_length.to_string());
+                return false;
+            }
+            return queried_length.absolute_length_to_px() >= value_length.absolute_length_to_px();
+        }
+        return false;
+
+    case Type::MaxValue:
+        if (queried_value->has_number() && value->has_number())
+            return queried_value->to_number() <= value->to_number();
+        if (queried_value->has_length() && value->has_length()) {
+            auto queried_length = queried_value->to_length();
+            auto value_length = value->to_length();
+            // FIXME: We should be checking that lengths are valid during parsing
+            if (!value_length.is_absolute()) {
+                dbgln("Media feature was given a non-absolute length, which is invalid! {}", value_length.to_string());
+                return false;
+            }
+            return queried_length.absolute_length_to_px() <= value_length.absolute_length_to_px();
+        }
+        return false;
+    }
+
+    VERIFY_NOT_REACHED();
+}
+
 String MediaQuery::MediaCondition::to_string() const
 {
     StringBuilder builder;
@@ -56,6 +111,33 @@ String MediaQuery::MediaCondition::to_string() const
     return builder.to_string();
 }
 
+bool MediaQuery::MediaCondition::evaluate(DOM::Window const& window) const
+{
+    switch (type) {
+    case Type::Single:
+        return feature.evaluate(window);
+
+    case Type::Not:
+        return !conditions.first().evaluate(window);
+
+    case Type::And:
+        for (auto& child : conditions) {
+            if (!child.evaluate(window))
+                return false;
+        }
+        return true;
+
+    case Type::Or:
+        for (auto& child : conditions) {
+            if (child.evaluate(window))
+                return true;
+        }
+        return false;
+    }
+
+    VERIFY_NOT_REACHED();
+}
+
 String MediaQuery::to_string() const
 {
     StringBuilder builder;
@@ -110,4 +192,39 @@ String MediaQuery::to_string() const
     return builder.to_string();
 }
 
+bool MediaQuery::evaluate(DOM::Window const& window)
+{
+    auto matches_media = [](MediaType media) -> bool {
+        switch (media) {
+        case MediaType::All:
+            return true;
+        case MediaType::Print:
+            // FIXME: Enable for printing, when we have printing!
+            return false;
+        case MediaType::Screen:
+            // FIXME: Disable for printing, when we have printing!
+            return true;
+        // Deprecated, must never match:
+        case MediaType::TTY:
+        case MediaType::TV:
+        case MediaType::Projection:
+        case MediaType::Handheld:
+        case MediaType::Braille:
+        case MediaType::Embossed:
+        case MediaType::Aural:
+        case MediaType::Speech:
+            return false;
+        }
+        VERIFY_NOT_REACHED();
+    };
+
+    bool result = matches_media(m_media_type);
+
+    if (result && m_media_condition)
+        result = m_media_condition->evaluate(window);
+
+    m_matches = m_negated ? !result : result;
+    return m_matches;
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/CSS/MediaQuery.h b/Userland/Libraries/LibWeb/CSS/MediaQuery.h
index 97775333c3..9321525227 100644
--- a/Userland/Libraries/LibWeb/CSS/MediaQuery.h
+++ b/Userland/Libraries/LibWeb/CSS/MediaQuery.h
@@ -53,6 +53,7 @@ public:
         FlyString name;
         RefPtr<StyleValue> value { nullptr };
 
+        bool evaluate(DOM::Window const&) const;
         String to_string() const;
     };
 
@@ -69,12 +70,15 @@ public:
         MediaFeature feature;
         NonnullOwnPtrVector<MediaCondition> conditions;
 
+        bool evaluate(DOM::Window const&) const;
         String to_string() const;
     };
 
     static NonnullRefPtr<MediaQuery> create_not_all();
     static NonnullRefPtr<MediaQuery> create() { return adopt_ref(*new MediaQuery); }
 
+    bool matches() const { return m_matches; }
+    bool evaluate(DOM::Window const&);
     String to_string() const;
 
 private:
@@ -84,6 +88,9 @@ private:
     bool m_negated { false };
     MediaType m_media_type { MediaType::All };
     OwnPtr<MediaCondition> m_media_condition { nullptr };
+
+    // Cached value, updated by evaluate()
+    bool m_matches { false };
 };
 
 }
diff --git a/Userland/Libraries/LibWeb/CSS/MediaQueryList.cpp b/Userland/Libraries/LibWeb/CSS/MediaQueryList.cpp
index d3295c3075..f581a96e96 100644
--- a/Userland/Libraries/LibWeb/CSS/MediaQueryList.cpp
+++ b/Userland/Libraries/LibWeb/CSS/MediaQueryList.cpp
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -18,6 +19,7 @@ MediaQueryList::MediaQueryList(DOM::Document& document, NonnullRefPtrVector<Medi
     , m_document(document)
     , m_media(move(media))
 {
+    evaluate();
 }
 
 MediaQueryList::~MediaQueryList()
@@ -35,10 +37,23 @@ String MediaQueryList::media() const
 // https://drafts.csswg.org/cssom-view/#dom-mediaquerylist-matches
 bool MediaQueryList::matches() const
 {
-    // TODO: Implement me :^)
+    for (auto& media : m_media) {
+        if (media.matches())
+            return true;
+    }
     return false;
 }
 
+bool MediaQueryList::evaluate()
+{
+    bool now_matches = false;
+    for (auto& media : m_media) {
+        now_matches = now_matches || media.evaluate(m_document.window());
+    }
+
+    return now_matches;
+}
+
 JS::Object* MediaQueryList::create_wrapper(JS::GlobalObject& global_object)
 {
     return wrap(global_object, *this);
diff --git a/Userland/Libraries/LibWeb/CSS/MediaQueryList.h b/Userland/Libraries/LibWeb/CSS/MediaQueryList.h
index 45c2caf099..ced43fe3d2 100644
--- a/Userland/Libraries/LibWeb/CSS/MediaQueryList.h
+++ b/Userland/Libraries/LibWeb/CSS/MediaQueryList.h
@@ -8,6 +8,7 @@
 
 #include <AK/Forward.h>
 #include <AK/RefCounted.h>
+#include <AK/Weakable.h>
 #include <LibWeb/Bindings/Wrappable.h>
 #include <LibWeb/CSS/MediaQuery.h>
 #include <LibWeb/DOM/EventTarget.h>
@@ -18,6 +19,7 @@ namespace Web::CSS {
 // 4.2. The MediaQueryList Interface, https://drafts.csswg.org/cssom-view/#the-mediaquerylist-interface
 class MediaQueryList final
     : public RefCounted<MediaQueryList>
+    , public Weakable<MediaQueryList>
     , public DOM::EventTarget
     , public Bindings::Wrappable {
 
@@ -36,6 +38,7 @@ public:
 
     String media() const;
     bool matches() const;
+    bool evaluate();
 
     // ^EventTarget
     virtual void ref_event_target() override { ref(); }