diff --git a/Libraries/LibWeb/CSS/StyleResolver.cpp b/Libraries/LibWeb/CSS/StyleResolver.cpp
index bb798b6c0d..513b319b1f 100644
--- a/Libraries/LibWeb/CSS/StyleResolver.cpp
+++ b/Libraries/LibWeb/CSS/StyleResolver.cpp
@@ -52,7 +52,7 @@ static StyleSheet& default_stylesheet()
     if (!sheet) {
         extern const char default_stylesheet_source[];
         String css = default_stylesheet_source;
-        sheet = parse_css(css).leak_ref();
+        sheet = parse_css(CSS::ParsingContext(), css).leak_ref();
     }
     return *sheet;
 }
@@ -205,6 +205,8 @@ static inline void set_property_border_style(StyleProperties& style, const Style
 
 static void set_property_expanding_shorthands(StyleProperties& style, CSS::PropertyID property_id, const StyleValue& value, Document& document)
 {
+    CSS::ParsingContext context(document);
+
     if (property_id == CSS::PropertyID::Border) {
         set_property_expanding_shorthands(style, CSS::PropertyID::BorderTop, value, document);
         set_property_expanding_shorthands(style, CSS::PropertyID::BorderRight, value, document);
@@ -248,7 +250,7 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
             auto parts = split_on_whitespace(value.to_string());
 
             if (parts.size() == 1) {
-                if (auto value = parse_line_style(parts[0])) {
+                if (auto value = parse_line_style(context, parts[0])) {
                     set_property_border_style(style, value.release_nonnull(), edge);
                     set_property_border_color(style, ColorStyleValue::create(Gfx::Color::Black), edge);
                     set_property_border_width(style, LengthStyleValue::create(Length(3, Length::Type::Px)), edge);
@@ -261,19 +263,19 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
             RefPtr<StringStyleValue> line_style_value;
 
             for (auto& part : parts) {
-                if (auto value = parse_line_width(part)) {
+                if (auto value = parse_line_width(context, part)) {
                     if (line_width_value)
                         return;
                     line_width_value = move(value);
                     continue;
                 }
-                if (auto value = parse_color(part)) {
+                if (auto value = parse_color(context, part)) {
                     if (color_value)
                         return;
                     color_value = move(value);
                     continue;
                 }
-                if (auto value = parse_line_style(part)) {
+                if (auto value = parse_line_style(context, part)) {
                     if (line_style_value)
                         return;
                     line_style_value = move(value);
@@ -296,10 +298,16 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
     if (property_id == CSS::PropertyID::BorderStyle) {
         auto parts = split_on_whitespace(value.to_string());
         if (value.is_string() && parts.size() == 3) {
-            style.set_property(CSS::PropertyID::BorderTopStyle, parse_css_value(parts[0]));
-            style.set_property(CSS::PropertyID::BorderRightStyle, parse_css_value(parts[1]));
-            style.set_property(CSS::PropertyID::BorderBottomStyle, parse_css_value(parts[2]));
-            style.set_property(CSS::PropertyID::BorderLeftStyle, parse_css_value(parts[1]));
+            auto top = parse_css_value(context, parts[0]);
+            auto right = parse_css_value(context, parts[1]);
+            auto bottom = parse_css_value(context, parts[2]);
+            auto left = parse_css_value(context, parts[1]);
+            if (top && right && bottom && left) {
+                style.set_property(CSS::PropertyID::BorderTopStyle, *top);
+                style.set_property(CSS::PropertyID::BorderRightStyle, *right);
+                style.set_property(CSS::PropertyID::BorderBottomStyle, *bottom);
+                style.set_property(CSS::PropertyID::BorderLeftStyle, *left);
+            }
         } else {
             style.set_property(CSS::PropertyID::BorderTopStyle, value);
             style.set_property(CSS::PropertyID::BorderRightStyle, value);
@@ -312,12 +320,14 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
     if (property_id == CSS::PropertyID::BorderWidth) {
         auto parts = split_on_whitespace(value.to_string());
         if (value.is_string() && parts.size() == 2) {
-            auto vertical_border_width = parse_css_value(parts[0]);
-            auto horizontal_border_width = parse_css_value(parts[1]);
-            style.set_property(CSS::PropertyID::BorderTopWidth, vertical_border_width);
-            style.set_property(CSS::PropertyID::BorderRightWidth, horizontal_border_width);
-            style.set_property(CSS::PropertyID::BorderBottomWidth, vertical_border_width);
-            style.set_property(CSS::PropertyID::BorderLeftWidth, horizontal_border_width);
+            auto vertical_border_width = parse_css_value(context, parts[0]);
+            auto horizontal_border_width = parse_css_value(context, parts[1]);
+            if (vertical_border_width && horizontal_border_width) {
+                style.set_property(CSS::PropertyID::BorderTopWidth, *vertical_border_width);
+                style.set_property(CSS::PropertyID::BorderRightWidth, *horizontal_border_width);
+                style.set_property(CSS::PropertyID::BorderBottomWidth, *vertical_border_width);
+                style.set_property(CSS::PropertyID::BorderLeftWidth, *horizontal_border_width);
+            }
         } else {
             style.set_property(CSS::PropertyID::BorderTopWidth, value);
             style.set_property(CSS::PropertyID::BorderRightWidth, value);
@@ -330,10 +340,16 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
     if (property_id == CSS::PropertyID::BorderColor) {
         auto parts = split_on_whitespace(value.to_string());
         if (value.is_string() && parts.size() == 4) {
-            style.set_property(CSS::PropertyID::BorderTopColor, parse_css_value(parts[0]));
-            style.set_property(CSS::PropertyID::BorderRightColor, parse_css_value(parts[1]));
-            style.set_property(CSS::PropertyID::BorderBottomColor, parse_css_value(parts[2]));
-            style.set_property(CSS::PropertyID::BorderLeftColor, parse_css_value(parts[3]));
+            auto top = parse_css_value(context, parts[0]);
+            auto right = parse_css_value(context, parts[1]);
+            auto bottom = parse_css_value(context, parts[2]);
+            auto left = parse_css_value(context, parts[3]);
+            if (top && right && bottom &&left) {
+                style.set_property(CSS::PropertyID::BorderTopColor, *top);
+                style.set_property(CSS::PropertyID::BorderRightColor, *right);
+                style.set_property(CSS::PropertyID::BorderBottomColor, *bottom);
+                style.set_property(CSS::PropertyID::BorderLeftColor, *left);
+            }
         } else {
             style.set_property(CSS::PropertyID::BorderTopColor, value);
             style.set_property(CSS::PropertyID::BorderRightColor, value);
@@ -351,7 +367,10 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
         auto parts = split_on_whitespace(value.to_string());
         NonnullRefPtrVector<StyleValue> values;
         for (auto& part : parts) {
-            values.append(parse_css_value(part));
+            auto value = parse_css_value(context, part);
+            if (!value)
+                return;
+            values.append(value.release_nonnull());
         }
 
         // HACK: Disallow more than one color value in a 'background' shorthand
@@ -393,33 +412,39 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
         if (value.is_string()) {
             auto parts = split_on_whitespace(value.to_string());
             if (value.is_string() && parts.size() == 2) {
-                auto vertical = parse_css_value(parts[0]);
-                auto horizontal = parse_css_value(parts[1]);
-                style.set_property(CSS::PropertyID::MarginTop, vertical);
-                style.set_property(CSS::PropertyID::MarginBottom, vertical);
-                style.set_property(CSS::PropertyID::MarginLeft, horizontal);
-                style.set_property(CSS::PropertyID::MarginRight, horizontal);
+                auto vertical = parse_css_value(context, parts[0]);
+                auto horizontal = parse_css_value(context, parts[1]);
+                if (vertical && horizontal) {
+                    style.set_property(CSS::PropertyID::MarginTop, *vertical);
+                    style.set_property(CSS::PropertyID::MarginBottom, *vertical);
+                    style.set_property(CSS::PropertyID::MarginLeft, *horizontal);
+                    style.set_property(CSS::PropertyID::MarginRight, *horizontal);
+                }
                 return;
             }
             if (value.is_string() && parts.size() == 3) {
-                auto top = parse_css_value(parts[0]);
-                auto horizontal = parse_css_value(parts[1]);
-                auto bottom = parse_css_value(parts[2]);
-                style.set_property(CSS::PropertyID::MarginTop, top);
-                style.set_property(CSS::PropertyID::MarginBottom, bottom);
-                style.set_property(CSS::PropertyID::MarginLeft, horizontal);
-                style.set_property(CSS::PropertyID::MarginRight, horizontal);
+                auto top = parse_css_value(context, parts[0]);
+                auto horizontal = parse_css_value(context, parts[1]);
+                auto bottom = parse_css_value(context, parts[2]);
+                if (top && horizontal && bottom) {
+                    style.set_property(CSS::PropertyID::MarginTop, *top);
+                    style.set_property(CSS::PropertyID::MarginBottom, *bottom);
+                    style.set_property(CSS::PropertyID::MarginLeft, *horizontal);
+                    style.set_property(CSS::PropertyID::MarginRight, *horizontal);
+                }
                 return;
             }
             if (value.is_string() && parts.size() == 4) {
-                auto top = parse_css_value(parts[0]);
-                auto right = parse_css_value(parts[1]);
-                auto bottom = parse_css_value(parts[2]);
-                auto left = parse_css_value(parts[3]);
-                style.set_property(CSS::PropertyID::MarginTop, top);
-                style.set_property(CSS::PropertyID::MarginBottom, bottom);
-                style.set_property(CSS::PropertyID::MarginLeft, left);
-                style.set_property(CSS::PropertyID::MarginRight, right);
+                auto top = parse_css_value(context, parts[0]);
+                auto right = parse_css_value(context, parts[1]);
+                auto bottom = parse_css_value(context, parts[2]);
+                auto left = parse_css_value(context, parts[3]);
+                if (top && right && bottom && left) {
+                    style.set_property(CSS::PropertyID::MarginTop, *top);
+                    style.set_property(CSS::PropertyID::MarginBottom, *bottom);
+                    style.set_property(CSS::PropertyID::MarginLeft, *left);
+                    style.set_property(CSS::PropertyID::MarginRight, *right);
+                }
                 return;
             }
             dbg() << "Unsure what to do with CSS margin value '" << value.to_string() << "'";
@@ -439,33 +464,39 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
         if (value.is_string()) {
             auto parts = split_on_whitespace(value.to_string());
             if (value.is_string() && parts.size() == 2) {
-                auto vertical = parse_css_value(parts[0]);
-                auto horizontal = parse_css_value(parts[1]);
-                style.set_property(CSS::PropertyID::PaddingTop, vertical);
-                style.set_property(CSS::PropertyID::PaddingBottom, vertical);
-                style.set_property(CSS::PropertyID::PaddingLeft, horizontal);
-                style.set_property(CSS::PropertyID::PaddingRight, horizontal);
+                auto vertical = parse_css_value(context, parts[0]);
+                auto horizontal = parse_css_value(context, parts[1]);
+                if (vertical && horizontal) {
+                    style.set_property(CSS::PropertyID::PaddingTop, *vertical);
+                    style.set_property(CSS::PropertyID::PaddingBottom, *vertical);
+                    style.set_property(CSS::PropertyID::PaddingLeft, *horizontal);
+                    style.set_property(CSS::PropertyID::PaddingRight, *horizontal);
+                }
                 return;
             }
             if (value.is_string() && parts.size() == 3) {
-                auto top = parse_css_value(parts[0]);
-                auto horizontal = parse_css_value(parts[1]);
-                auto bottom = parse_css_value(parts[2]);
-                style.set_property(CSS::PropertyID::PaddingTop, top);
-                style.set_property(CSS::PropertyID::PaddingBottom, bottom);
-                style.set_property(CSS::PropertyID::PaddingLeft, horizontal);
-                style.set_property(CSS::PropertyID::PaddingRight, horizontal);
+                auto top = parse_css_value(context, parts[0]);
+                auto horizontal = parse_css_value(context, parts[1]);
+                auto bottom = parse_css_value(context, parts[2]);
+                if (top && bottom && horizontal) {
+                    style.set_property(CSS::PropertyID::PaddingTop, *top);
+                    style.set_property(CSS::PropertyID::PaddingBottom, *bottom);
+                    style.set_property(CSS::PropertyID::PaddingLeft, *horizontal);
+                    style.set_property(CSS::PropertyID::PaddingRight, *horizontal);
+                }
                 return;
             }
             if (value.is_string() && parts.size() == 4) {
-                auto top = parse_css_value(parts[0]);
-                auto right = parse_css_value(parts[1]);
-                auto bottom = parse_css_value(parts[2]);
-                auto left = parse_css_value(parts[3]);
-                style.set_property(CSS::PropertyID::PaddingTop, top);
-                style.set_property(CSS::PropertyID::PaddingBottom, bottom);
-                style.set_property(CSS::PropertyID::PaddingLeft, left);
-                style.set_property(CSS::PropertyID::PaddingRight, right);
+                auto top = parse_css_value(context, parts[0]);
+                auto right = parse_css_value(context, parts[1]);
+                auto bottom = parse_css_value(context, parts[2]);
+                auto left = parse_css_value(context, parts[3]);
+                if (top && bottom && left && right) {
+                    style.set_property(CSS::PropertyID::PaddingTop, *top);
+                    style.set_property(CSS::PropertyID::PaddingBottom, *bottom);
+                    style.set_property(CSS::PropertyID::PaddingLeft, *left);
+                    style.set_property(CSS::PropertyID::PaddingRight, *right);
+                }
                 return;
             }
             dbg() << "Unsure what to do with CSS padding value '" << value.to_string() << "'";
@@ -477,8 +508,10 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
     if (property_id == CSS::PropertyID::ListStyle) {
         auto parts = split_on_whitespace(value.to_string());
         if (!parts.is_empty()) {
-            auto value = parse_css_value(parts[0]);
-            style.set_property(CSS::PropertyID::ListStyleType, value);
+            auto value = parse_css_value(context, parts[0]);
+            if (!value)
+                return;
+            style.set_property(CSS::PropertyID::ListStyleType, value.release_nonnull());
         }
         return;
     }
@@ -522,7 +555,7 @@ NonnullRefPtr<StyleProperties> StyleResolver::resolve_style(const Element& eleme
 
     auto style_attribute = element.attribute(HTML::AttributeNames::style);
     if (!style_attribute.is_null()) {
-        if (auto declaration = parse_css_declaration(style_attribute)) {
+        if (auto declaration = parse_css_declaration(CSS::ParsingContext(document()), style_attribute)) {
             for (auto& property : declaration->properties()) {
                 set_property_expanding_shorthands(style, property.property_id, property.value, m_document);
             }
diff --git a/Libraries/LibWeb/DOM/Document.cpp b/Libraries/LibWeb/DOM/Document.cpp
index 93f27062ee..fb4bcd838f 100644
--- a/Libraries/LibWeb/DOM/Document.cpp
+++ b/Libraries/LibWeb/DOM/Document.cpp
@@ -346,7 +346,7 @@ NonnullRefPtrVector<Element> Document::get_elements_by_tag_name(const String& ta
 
 RefPtr<Element> Document::query_selector(const StringView& selector_text)
 {
-    auto selector = parse_selector(selector_text);
+    auto selector = parse_selector(CSS::ParsingContext(*this), selector_text);
     if (!selector.has_value())
         return {};
 
@@ -366,7 +366,7 @@ RefPtr<Element> Document::query_selector(const StringView& selector_text)
 
 NonnullRefPtrVector<Element> Document::query_selector_all(const StringView& selector_text)
 {
-    auto selector = parse_selector(selector_text);
+    auto selector = parse_selector(CSS::ParsingContext(*this), selector_text);
     if (!selector.has_value())
         return {};
 
diff --git a/Libraries/LibWeb/DOM/HTMLLinkElement.cpp b/Libraries/LibWeb/DOM/HTMLLinkElement.cpp
index 40779cafde..f82d188128 100644
--- a/Libraries/LibWeb/DOM/HTMLLinkElement.cpp
+++ b/Libraries/LibWeb/DOM/HTMLLinkElement.cpp
@@ -63,7 +63,7 @@ void HTMLLinkElement::resource_did_load()
 
     dbg() << "HTMLLinkElement: Resource did load, looks good! " << href();
 
-    auto sheet = parse_css(resource()->encoded_data());
+    auto sheet = parse_css(CSS::ParsingContext(document()), resource()->encoded_data());
     if (!sheet) {
         dbg() << "HTMLLinkElement: Failed to parse stylesheet: " << href();
         return;
diff --git a/Libraries/LibWeb/DOM/HTMLStyleElement.cpp b/Libraries/LibWeb/DOM/HTMLStyleElement.cpp
index e2e2ec4a27..0ab7a6be20 100644
--- a/Libraries/LibWeb/DOM/HTMLStyleElement.cpp
+++ b/Libraries/LibWeb/DOM/HTMLStyleElement.cpp
@@ -48,7 +48,7 @@ void HTMLStyleElement::children_changed()
         if (is<Text>(child))
             builder.append(to<Text>(child).text_content());
     });
-    m_stylesheet = parse_css(builder.to_string());
+    m_stylesheet = parse_css(CSS::ParsingContext(document()), builder.to_string());
     if (m_stylesheet)
         document().style_sheets().add_sheet(*m_stylesheet);
     else
diff --git a/Libraries/LibWeb/DOM/HTMLTableElement.cpp b/Libraries/LibWeb/DOM/HTMLTableElement.cpp
index d36b165d8b..363001b7db 100644
--- a/Libraries/LibWeb/DOM/HTMLTableElement.cpp
+++ b/Libraries/LibWeb/DOM/HTMLTableElement.cpp
@@ -42,7 +42,8 @@ void HTMLTableElement::apply_presentational_hints(StyleProperties& style) const
 {
     for_each_attribute([&](auto& name, auto& value) {
         if (name == HTML::AttributeNames::width) {
-            style.set_property(CSS::PropertyID::Width, parse_css_value(value));
+            if (auto parsed_value = parse_css_value(CSS::ParsingContext(document()), value))
+                style.set_property(CSS::PropertyID::Width, parsed_value.release_nonnull());
             return;
         }
         if (name == HTML::AttributeNames::bgcolor) {
diff --git a/Libraries/LibWeb/Parser/CSSParser.cpp b/Libraries/LibWeb/Parser/CSSParser.cpp
index 3212701032..f22d69cda1 100644
--- a/Libraries/LibWeb/Parser/CSSParser.cpp
+++ b/Libraries/LibWeb/Parser/CSSParser.cpp
@@ -27,6 +27,7 @@
 #include <AK/HashMap.h>
 #include <LibWeb/CSS/PropertyID.h>
 #include <LibWeb/CSS/StyleSheet.h>
+#include <LibWeb/DOM/Document.h>
 #include <LibWeb/Parser/CSSParser.h>
 #include <ctype.h>
 #include <stdio.h>
@@ -46,6 +47,24 @@
 
 namespace Web {
 
+namespace CSS {
+
+ParsingContext::ParsingContext()
+{
+}
+
+ParsingContext::ParsingContext(const Document& document)
+    : m_document(&document)
+{
+}
+
+bool ParsingContext::in_quirks_mode() const
+{
+    return m_document ? m_document->in_quirks_mode() : false;
+}
+
+}
+
 static CSS::ValueID value_id_for_palette_string(const StringView& string)
 {
     if (string == "desktop-background")
@@ -160,7 +179,7 @@ static CSS::ValueID value_id_for_palette_string(const StringView& string)
         return CSS::ValueID::Invalid;
 }
 
-static Optional<Color> parse_css_color(const StringView& view)
+static Optional<Color> parse_css_color(const CSS::ParsingContext&, const StringView& view)
 {
     if (view.equals_ignoring_case("transparent"))
         return Color::from_rgba(0x00000000);
@@ -245,35 +264,72 @@ static Optional<float> try_parse_float(const StringView& string)
     return is_negative ? -value : value;
 }
 
-static Optional<float> parse_number(const StringView& view)
+static Optional<float> parse_number(const CSS::ParsingContext& context, const StringView& view)
 {
     if (view.ends_with('%'))
-        return parse_number(view.substring_view(0, view.length() - 1));
+        return parse_number(context, view.substring_view(0, view.length() - 1));
 
     // FIXME: Maybe we should have "ends_with_ignoring_case()" ?
     if (view.to_string().to_lowercase().ends_with("px"))
-        return parse_number(view.substring_view(0, view.length() - 2));
+        return parse_number(context, view.substring_view(0, view.length() - 2));
     if (view.to_string().to_lowercase().ends_with("rem"))
-        return parse_number(view.substring_view(0, view.length() - 3));
+        return parse_number(context, view.substring_view(0, view.length() - 3));
     if (view.to_string().to_lowercase().ends_with("em"))
-        return parse_number(view.substring_view(0, view.length() - 2));
+        return parse_number(context, view.substring_view(0, view.length() - 2));
+    if (view == "0")
+        return 0;
+
+    // NOTE: We don't allow things like "width: 100" in standards mode.
+    if (!context.in_quirks_mode())
+        return {};
 
     return try_parse_float(view);
 }
 
-NonnullRefPtr<StyleValue> parse_css_value(const StringView& string)
+static Length parse_length(const CSS::ParsingContext& context, const StringView& view, bool& is_bad_length)
 {
-    auto number = parse_number(string);
-    if (number.has_value()) {
-        if (string.ends_with('%'))
-            return LengthStyleValue::create(Length(number.value(), Length::Type::Percentage));
-        if (string.ends_with("em"))
-            return LengthStyleValue::create(Length(number.value(), Length::Type::Em));
-        if (string.ends_with("rem"))
-            return LengthStyleValue::create(Length(number.value(), Length::Type::Rem));
-        return LengthStyleValue::create(Length(number.value(), Length::Type::Px));
+    Length::Type type = Length::Type::Undefined;
+    Optional<float> value;
+
+    if (view.ends_with('%')) {
+        type = Length::Type::Percentage;
+        value = try_parse_float(view.substring_view(0, view.length() - 1));
+    } else if (view.to_string().to_lowercase().ends_with("px")) {
+        type = Length::Type::Px;
+        value = try_parse_float(view.substring_view(0, view.length() - 2));
+    } else if (view.to_string().to_lowercase().ends_with("rem")) {
+        type = Length::Type::Rem;
+        value = try_parse_float(view.substring_view(0, view.length() - 3));
+    } else if (view.to_string().to_lowercase().ends_with("em")) {
+        type = Length::Type::Em;
+        value = try_parse_float(view.substring_view(0, view.length() - 2));
+    } else if (view == "0") {
+        type = Length::Type::Px;
+        value = 0;
+    } else if (context.in_quirks_mode()) {
+        type = Length::Type::Px;
+        value = try_parse_float(view);
+    } else {
+        value = try_parse_float(view);
+        if (value.has_value())
+            is_bad_length = true;
     }
 
+    if (!value.has_value())
+        return {};
+
+    return Length(value.value(), type);
+}
+
+RefPtr<StyleValue> parse_css_value(const CSS::ParsingContext& context, const StringView& string)
+{
+    bool is_bad_length = false;
+    auto length = parse_length(context, string, is_bad_length);
+    if (is_bad_length)
+        return nullptr;
+    if (!length.is_undefined())
+        return LengthStyleValue::create(length);
+
     if (string.equals_ignoring_case("inherit"))
         return InheritStyleValue::create();
     if (string.equals_ignoring_case("initial"))
@@ -281,7 +337,7 @@ NonnullRefPtr<StyleValue> parse_css_value(const StringView& string)
     if (string.equals_ignoring_case("auto"))
         return LengthStyleValue::create(Length::make_auto());
 
-    auto color = parse_css_color(string);
+    auto color = parse_css_color(context, string);
     if (color.has_value())
         return ColorStyleValue::create(color.value());
 
@@ -295,26 +351,26 @@ NonnullRefPtr<StyleValue> parse_css_value(const StringView& string)
     return StringStyleValue::create(string);
 }
 
-RefPtr<LengthStyleValue> parse_line_width(const StringView& part)
+RefPtr<LengthStyleValue> parse_line_width(const CSS::ParsingContext& context, const StringView& part)
 {
-    NonnullRefPtr<StyleValue> value = parse_css_value(part);
-    if (value->is_length())
+    auto value = parse_css_value(context, part);
+    if (value && value->is_length())
         return static_ptr_cast<LengthStyleValue>(value);
     return nullptr;
 }
 
-RefPtr<ColorStyleValue> parse_color(const StringView& part)
+RefPtr<ColorStyleValue> parse_color(const CSS::ParsingContext& context, const StringView& part)
 {
-    NonnullRefPtr<StyleValue> value = parse_css_value(part);
-    if (value->is_color())
+    auto value = parse_css_value(context, part);
+    if (value && value->is_color())
         return static_ptr_cast<ColorStyleValue>(value);
     return nullptr;
 }
 
-RefPtr<StringStyleValue> parse_line_style(const StringView& part)
+RefPtr<StringStyleValue> parse_line_style(const CSS::ParsingContext& context, const StringView& part)
 {
-    NonnullRefPtr<StyleValue> parsed_value = parse_css_value(part);
-    if (!parsed_value->is_string())
+    auto parsed_value = parse_css_value(context, part);
+    if (!parsed_value || !parsed_value->is_string())
         return nullptr;
     auto value = static_ptr_cast<StringStyleValue>(parsed_value);
     if (value->to_string() == "dotted")
@@ -334,8 +390,9 @@ RefPtr<StringStyleValue> parse_line_style(const StringView& part)
 
 class CSSParser {
 public:
-    CSSParser(const StringView& input)
-        : css(input)
+    CSSParser(const CSS::ParsingContext& context, const StringView& input)
+        : m_context(context)
+        , css(input)
     {
     }
 
@@ -769,7 +826,10 @@ public:
         if (property_id == CSS::PropertyID::Invalid) {
             dbg() << "CSSParser: Unrecognized property '" << property_name << "'";
         }
-        return StyleProperty { property_id, parse_css_value(property_value), important };
+        auto value = parse_css_value(m_context, property_value);
+        if (!value)
+            return {};
+        return StyleProperty { property_id, value.release_nonnull(), important };
     }
 
     void parse_declaration()
@@ -841,6 +901,8 @@ public:
     }
 
 private:
+    CSS::ParsingContext m_context;
+
     NonnullRefPtrVector<StyleRule> rules;
 
     struct CurrentRule {
@@ -856,25 +918,25 @@ private:
     StringView css;
 };
 
-Optional<Selector> parse_selector(const StringView& selector_text)
+Optional<Selector> parse_selector(const CSS::ParsingContext& context, const StringView& selector_text)
 {
-    CSSParser parser(selector_text);
+    CSSParser parser(context, selector_text);
     return parser.parse_individual_selector();
 }
 
-RefPtr<StyleSheet> parse_css(const StringView& css)
+RefPtr<StyleSheet> parse_css(const CSS::ParsingContext& context, const StringView& css)
 {
     if (css.is_empty())
         return StyleSheet::create({});
-    CSSParser parser(css);
+    CSSParser parser(context, css);
     return parser.parse_sheet();
 }
 
-RefPtr<StyleDeclaration> parse_css_declaration(const StringView& css)
+RefPtr<StyleDeclaration> parse_css_declaration(const CSS::ParsingContext& context, const StringView& css)
 {
     if (css.is_empty())
         return StyleDeclaration::create({});
-    CSSParser parser(css);
+    CSSParser parser(context, css);
     return parser.parse_standalone_declaration();
 }
 
diff --git a/Libraries/LibWeb/Parser/CSSParser.h b/Libraries/LibWeb/Parser/CSSParser.h
index 1ec3209740..25f478c16f 100644
--- a/Libraries/LibWeb/Parser/CSSParser.h
+++ b/Libraries/LibWeb/Parser/CSSParser.h
@@ -31,13 +31,26 @@
 
 namespace Web {
 
-RefPtr<StyleSheet> parse_css(const StringView&);
-RefPtr<StyleDeclaration> parse_css_declaration(const StringView&);
-NonnullRefPtr<StyleValue> parse_css_value(const StringView&);
-Optional<Selector> parse_selector(const StringView&);
+namespace CSS {
+class ParsingContext {
+public:
+    ParsingContext();
+    explicit ParsingContext(const Document&);
 
-RefPtr<LengthStyleValue> parse_line_width(const StringView&);
-RefPtr<ColorStyleValue> parse_color(const StringView&);
-RefPtr<StringStyleValue> parse_line_style(const StringView&);
+    bool in_quirks_mode() const;
+
+private:
+    const Document* m_document { nullptr };
+};
+}
+
+RefPtr<StyleSheet> parse_css(const CSS::ParsingContext&, const StringView&);
+RefPtr<StyleDeclaration> parse_css_declaration(const CSS::ParsingContext&, const StringView&);
+RefPtr<StyleValue> parse_css_value(const CSS::ParsingContext&, const StringView&);
+Optional<Selector> parse_selector(const CSS::ParsingContext&, const StringView&);
+
+RefPtr<LengthStyleValue> parse_line_width(const CSS::ParsingContext&, const StringView&);
+RefPtr<ColorStyleValue> parse_color(const CSS::ParsingContext&, const StringView&);
+RefPtr<StringStyleValue> parse_line_style(const CSS::ParsingContext&, const StringView&);
 
 }