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LibWeb/CSS: Reimplement var()/attr() as arbitrary substitution functions

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"Arbitrary substitution functions" are a family of functions that
includes var() and attr(). All of them resolve to an arbitrary set of
component values that are not known at parse-time, so they have to be
substituted at computed-value time.

Besides it being nice to follow the spec closely, this means we'll be
able to implement the others (such as `if()` and `inherit()`) more
easily.

The main omission here is the new "spread syntax", which can be
implemented in the future.
This commit is contained in:
Sam Atkins 2025-06-19 17:03:26 +01:00 • committed by Tim Ledbetter
commit b6032b0fcd
Notes: github-actions[bot] 2025-07-09 15:45:46 +00:00
11 changed files with 480 additions and 346 deletions
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369
Libraries/LibWeb/CSS/Parser/ArbitrarySubstitutionFunctions.cpp Normal file
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@ -0,0 +1,369 @@
/*
* Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#include <LibWeb/CSS/Parser/ArbitrarySubstitutionFunctions.h>
#include <LibWeb/CSS/Parser/Parser.h>
#include <LibWeb/CSS/PropertyName.h>
#include <LibWeb/CSS/StyleComputer.h>
#include <LibWeb/CSS/StyleValues/UnresolvedStyleValue.h>
#include <LibWeb/DOM/Element.h>
namespace Web::CSS::Parser {
bool SubstitutionContext::operator==(SubstitutionContext const& other) const
{
return dependency_type == other.dependency_type
&& first == other.first
&& second == other.second;
}
String SubstitutionContext::to_string() const
{
StringView type_name = [this] {
switch (dependency_type) {
case DependencyType::Property:
return "Property"sv;
case DependencyType::Attribute:
return "Attribute"sv;
case DependencyType::Function:
return "Function"sv;
}
VERIFY_NOT_REACHED();
}();
return MUST(String::formatted("{} {} {}", type_name, first, second));
}
void GuardedSubstitutionContexts::guard(SubstitutionContext& context)
{
for (auto& existing_context : m_contexts) {
if (existing_context == context) {
existing_context.is_cyclic = true;
context.is_cyclic = true;
return;
}
}
m_contexts.append(context);
}
void GuardedSubstitutionContexts::unguard(SubstitutionContext const& context)
{
[[maybe_unused]] auto const was_removed = m_contexts.remove_first_matching([context](auto const& other) {
return context == other;
});
VERIFY(was_removed);
}
Optional<ArbitrarySubstitutionFunction> to_arbitrary_substitution_function(FlyString const& name)
{
if (name.equals_ignoring_ascii_case("attr"sv))
return ArbitrarySubstitutionFunction::Attr;
if (name.equals_ignoring_ascii_case("var"sv))
return ArbitrarySubstitutionFunction::Var;
return {};
}
bool contains_guaranteed_invalid_value(Vector<ComponentValue> const& values)
{
for (auto const& value : values) {
if (value.contains_guaranteed_invalid_value())
return true;
}
return false;
}
// https://drafts.csswg.org/css-values-5/#replace-an-attr-function
static Vector<ComponentValue> replace_an_attr_function(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, ArbitrarySubstitutionFunctionArguments const& arguments)
{
// 1. Let el be the element that the style containing the attr() function is being applied to.
// Let first arg be the first <declaration-value> in arguments.
// Let second arg be the <declaration-value>? passed after the comma, or null if there was no comma.
auto const& first_argument = arguments.first();
auto const second_argument = arguments.get(1);
FlyString attribute_name;
Optional<FlyString> maybe_syntax = {};
auto failure = [&] -> Vector<ComponentValue> {
// This is step 6, but defined here for convenience.
// 1. If second arg is null, and syntax was omitted, return an empty CSS <string>.
if (!second_argument.has_value() && !maybe_syntax.has_value())
return { Token::create_string({}) };
// 2. If second arg is null, return the guaranteed-invalid value.
if (!second_argument.has_value())
return { ComponentValue { GuaranteedInvalidValue {} } };
// 3. Substitute arbitrary substitution functions in second arg, and return the result.
return substitute_arbitrary_substitution_functions(element, guarded_contexts, second_argument.value());
};
// 2. Substitute arbitrary substitution functions in first arg, then parse it as <attr-name> <attr-type>?.
// If that returns failure, jump to the last step (labeled FAILURE).
// Otherwise, let attr name and syntax be the results of parsing (with syntax being null if <attr-type> was
// omitted), processed as specified in the definition of those arguments.
auto substituted = substitute_arbitrary_substitution_functions(element, guarded_contexts, first_argument);
TokenStream first_argument_tokens { substituted };
// <attr-name> = [ <ident-token>? '|' ]? <ident-token>
// FIXME: Support optional attribute namespace
if (!first_argument_tokens.next_token().is(Token::Type::Ident))
return failure();
attribute_name = first_argument_tokens.consume_a_token().token().ident();
first_argument_tokens.discard_whitespace();
// <attr-type> = type( <syntax> ) | raw-string | <attr-unit>
// FIXME: Support type(<syntax>)
bool is_dimension_unit = false;
if (first_argument_tokens.next_token().is(Token::Type::Ident)) {
auto const& syntax_ident = first_argument_tokens.next_token().token().ident();
if (syntax_ident.equals_ignoring_ascii_case("raw-string"sv)) {
maybe_syntax = first_argument_tokens.consume_a_token().token().ident();
} else {
is_dimension_unit = syntax_ident == "%"sv
|| Angle::unit_from_name(syntax_ident).has_value()
|| Flex::unit_from_name(syntax_ident).has_value()
|| Frequency::unit_from_name(syntax_ident).has_value()
|| Length::unit_from_name(syntax_ident).has_value()
|| Resolution::unit_from_name(syntax_ident).has_value()
|| Time::unit_from_name(syntax_ident).has_value();
if (is_dimension_unit)
maybe_syntax = first_argument_tokens.consume_a_token().token().ident();
}
}
first_argument_tokens.discard_whitespace();
if (first_argument_tokens.has_next_token())
return failure();
// 3. If attr name exists as an attribute on el, let attr value be its value; otherwise jump to the last step (labeled FAILURE).
// FIXME: Attribute namespaces
auto attribute_value = element.element().get_attribute(attribute_name);
if (!attribute_value.has_value())
return failure();
// 4. If syntax is null or the keyword raw-string, return a CSS <string> whose value is attr value.
// NOTE: No parsing or modification of any kind is performed on the value.
if (!maybe_syntax.has_value() || maybe_syntax->equals_ignoring_ascii_case("raw-string"sv))
return { Token::create_string(*attribute_value) };
auto syntax = maybe_syntax.release_value();
// 5. Substitute arbitrary substitution functions in attr value, with «"attribute", attr name» as the substitution
// context, then parse with a attr value, with syntax and el. If that succeeds, return the result; otherwise,
// jump to the last step (labeled FAILURE).
auto parser = Parser::create(ParsingParams { element.element().document() }, attribute_value.value());
auto unsubstituted_values = parser.parse_as_list_of_component_values();
auto substituted_values = substitute_arbitrary_substitution_functions(element, guarded_contexts, unsubstituted_values, SubstitutionContext { SubstitutionContext::DependencyType::Attribute, attribute_name.to_string() });
// FIXME: Parse using the syntax. For now we just handle `<attr-unit>` here.
TokenStream value_tokens { substituted_values };
value_tokens.discard_whitespace();
auto const& component_value = value_tokens.consume_a_token();
value_tokens.discard_whitespace();
if (value_tokens.has_next_token())
return failure();
if (component_value.is(Token::Type::Number) && is_dimension_unit)
return { Token::create_dimension(component_value.token().number_value(), syntax) };
return failure();
// 6. FAILURE:
// NB: Step 6 is a lambda defined at the top of the function.
}
// https://drafts.csswg.org/css-variables-1/#replace-a-var-function
static Vector<ComponentValue> replace_a_var_function(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, ArbitrarySubstitutionFunctionArguments const& arguments)
{
// 1. Let el be the element that the style containing the var() function is being applied to.
// Let first arg be the first <declaration-value> in arguments.
// Let second arg be the <declaration-value>? passed after the comma, or null if there was no comma.
auto const& first_argument = arguments.first();
auto const second_argument = arguments.get(1);
// 2. Substitute arbitrary substitution functions in first arg, then parse it as a <custom-property-name>.
// If parsing returned a <custom-property-name>, let result be the computed value of the corresponding custom
// property on el. Otherwise, let result be the guaranteed-invalid value.
auto substituted_first_argument = substitute_arbitrary_substitution_functions(element, guarded_contexts, first_argument);
TokenStream name_tokens { substituted_first_argument };
name_tokens.discard_whitespace();
auto& name_token = name_tokens.consume_a_token();
name_tokens.discard_whitespace();
Vector<ComponentValue> result;
if (name_tokens.has_next_token() || !name_token.is(Token::Type::Ident) || !is_a_custom_property_name_string(name_token.token().ident())) {
result = { ComponentValue { GuaranteedInvalidValue {} } };
} else {
// Look up the value of the custom property
auto& custom_property_name = name_token.token().ident();
auto custom_property_value = StyleComputer::compute_value_of_custom_property(element, custom_property_name, guarded_contexts);
if (custom_property_value->is_guaranteed_invalid()) {
result = { ComponentValue { GuaranteedInvalidValue {} } };
} else if (custom_property_value->is_unresolved()) {
result = custom_property_value->as_unresolved().values();
} else {
dbgln_if(CSS_PARSER_DEBUG, "Custom property `{}` is an unsupported type: {}", custom_property_name, to_underlying(custom_property_value->type()));
result = { ComponentValue { GuaranteedInvalidValue {} } };
}
}
// FIXME: 3. If the custom property named by the var()’s first argument is animation-tainted, and the var() is being used
// in a property that is not animatable, set result to the guaranteed-invalid value.
// 4. If result contains the guaranteed-invalid value, and second arg was provided, set result to the result of substitute arbitrary substitution functions on second arg.
if (contains_guaranteed_invalid_value(result) && second_argument.has_value())
result = substitute_arbitrary_substitution_functions(element, guarded_contexts, second_argument.value());
// 5. Return result.
return result;
}
static void substitute_arbitrary_substitution_functions_step_2(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, TokenStream<ComponentValue>& source, Vector<ComponentValue>& dest)
{
// Step 2 of https://drafts.csswg.org/css-values-5/#substitute-arbitrary-substitution-function
// 2. For each arbitrary substitution function func in values (ordered via a depth-first pre-order traversal) that
// is not nested in the contents of another arbitrary substitution function:
while (source.has_next_token()) {
auto const& value = source.consume_a_token();
if (value.is_function()) {
auto const& source_function = value.function();
if (auto maybe_function_id = to_arbitrary_substitution_function(source_function.name); maybe_function_id.has_value()) {
auto function_id = maybe_function_id.release_value();
// FIXME: 1. Substitute early-invoked functions in func’s contents, and let early result be the result.
auto const& early_result = source_function.value;
// 2. If early result contains the guaranteed-invalid value, replace func in values with the guaranteed-invalid
// value and continue.
if (contains_guaranteed_invalid_value(early_result)) {
dest.empend(GuaranteedInvalidValue {});
continue;
}
// 3. Parse early result according to func’s argument grammar. If this returns failure, replace func in values
// with the guaranteed-invalid value and continue; otherwise, let arguments be the result.
auto maybe_arguments = parse_according_to_argument_grammar(function_id, early_result);
if (!maybe_arguments.has_value()) {
dest.empend(GuaranteedInvalidValue {});
continue;
}
auto arguments = maybe_arguments.release_value();
// 4. Replace an arbitrary substitution function for func, given arguments, as defined by that function.
// Let result be the returned list of component values.
auto result = replace_an_arbitrary_substitution_function(element, guarded_contexts, function_id, arguments);
// 5. If result contains the guaranteed-invalid value, replace func in values with the guaranteed-invalid value.
// Otherwise, replace func in values with result.
if (contains_guaranteed_invalid_value(result)) {
dest.empend(GuaranteedInvalidValue {});
} else {
// NB: Because we're doing this in one pass recursively, we now need to substitute any ASFs in result.
TokenStream result_stream { result };
Vector<ComponentValue> result_after_processing;
substitute_arbitrary_substitution_functions_step_2(element, guarded_contexts, result_stream, result_after_processing);
dest.extend(result_after_processing);
}
continue;
}
Vector<ComponentValue> function_values;
TokenStream source_function_contents { source_function.value };
substitute_arbitrary_substitution_functions_step_2(element, guarded_contexts, source_function_contents, function_values);
dest.empend(Function { source_function.name, move(function_values) });
continue;
}
if (value.is_block()) {
auto const& source_block = value.block();
TokenStream source_block_values { source_block.value };
Vector<ComponentValue> block_values;
substitute_arbitrary_substitution_functions_step_2(element, guarded_contexts, source_block_values, block_values);
dest.empend(SimpleBlock { source_block.token, move(block_values) });
continue;
}
dest.empend(value);
}
}
// https://drafts.csswg.org/css-values-5/#substitute-arbitrary-substitution-function
Vector<ComponentValue> substitute_arbitrary_substitution_functions(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, Vector<ComponentValue> const& values, Optional<SubstitutionContext> context)
{
// To substitute arbitrary substitution functions in a sequence of component values values, given an optional
// substitution context context:
// 1. Guard context for the remainder of this algorithm. If context is marked as a cyclic substitution context,
// return the guaranteed-invalid value.
if (context.has_value()) {
guarded_contexts.guard(context.value());
if (context->is_cyclic)
return { ComponentValue { GuaranteedInvalidValue {} } };
}
ScopeGuard const guard { [&] {
if (context.has_value())
guarded_contexts.unguard(context.value());
} };
// 2. For each arbitrary substitution function func in values (ordered via a depth-first pre-order traversal) that
// is not nested in the contents of another arbitrary substitution function:
Vector<ComponentValue> new_values;
TokenStream source { values };
substitute_arbitrary_substitution_functions_step_2(element, guarded_contexts, source, new_values);
// 3. If context is marked as a cyclic substitution context, return the guaranteed-invalid value.
// NOTE: Nested arbitrary substitution functions may have marked context as cyclic in step 2.
if (context.has_value() && context->is_cyclic)
return { ComponentValue { GuaranteedInvalidValue {} } };
// 4. Return values.
return new_values;
}
Optional<ArbitrarySubstitutionFunctionArguments> parse_according_to_argument_grammar(ArbitrarySubstitutionFunction function, Vector<ComponentValue> const& values)
{
// Equivalent to `<declaration-value> , <declaration-value>?`, used by multiple argument grammars.
auto parse_declaration_value_then_optional_declaration_value = [](Vector<ComponentValue> const& values) -> Optional<ArbitrarySubstitutionFunctionArguments> {
TokenStream tokens { values };
auto first_argument = Parser::parse_declaration_value(tokens, Parser::StopAtComma::Yes);
if (!first_argument.has_value())
return OptionalNone {};
if (!tokens.has_next_token())
return ArbitrarySubstitutionFunctionArguments { first_argument.release_value() };
VERIFY(tokens.next_token().is(Token::Type::Comma));
tokens.discard_a_token(); // ,
auto second_argument = Parser::parse_declaration_value(tokens, Parser::StopAtComma::No);
if (tokens.has_next_token())
return OptionalNone {};
return ArbitrarySubstitutionFunctionArguments { first_argument.release_value(), second_argument.value_or({}) };
};
switch (function) {
case ArbitrarySubstitutionFunction::Attr:
// https://drafts.csswg.org/css-values-5/#attr-notation
// <attr-args> = attr( <declaration-value> , <declaration-value>? )
return parse_declaration_value_then_optional_declaration_value(values);
case ArbitrarySubstitutionFunction::Var:
// https://drafts.csswg.org/css-variables/#funcdef-var
// <var-args> = var( <declaration-value> , <declaration-value>? )
return parse_declaration_value_then_optional_declaration_value(values);
}
VERIFY_NOT_REACHED();
}
// https://drafts.csswg.org/css-values-5/#replace-an-arbitrary-substitution-function
Vector<ComponentValue> replace_an_arbitrary_substitution_function(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, ArbitrarySubstitutionFunction function, ArbitrarySubstitutionFunctionArguments const& arguments)
{
switch (function) {
case ArbitrarySubstitutionFunction::Attr:
return replace_an_attr_function(element, guarded_contexts, arguments);
case ArbitrarySubstitutionFunction::Var:
return replace_a_var_function(element, guarded_contexts, arguments);
}
VERIFY_NOT_REACHED();
}
}

54
Libraries/LibWeb/CSS/Parser/ArbitrarySubstitutionFunctions.h Normal file
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@ -0,0 +1,54 @@
/*
* Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include <LibWeb/CSS/Parser/Parser.h>
namespace Web::CSS::Parser {
// https://drafts.csswg.org/css-values-5/#substitution-context
struct SubstitutionContext {
enum class DependencyType : u8 {
Property,
Attribute,
Function,
};
DependencyType dependency_type;
String first;
Optional<String> second {};
bool is_cyclic { false };
bool operator==(SubstitutionContext const&) const;
String to_string() const;
};
class GuardedSubstitutionContexts {
public:
void guard(SubstitutionContext&);
void unguard(SubstitutionContext const&);
private:
Vector<SubstitutionContext&> m_contexts;
};
enum class ArbitrarySubstitutionFunction : u8 {
Attr,
Var,
};
[[nodiscard]] Optional<ArbitrarySubstitutionFunction> to_arbitrary_substitution_function(FlyString const& name);
bool contains_guaranteed_invalid_value(Vector<ComponentValue> const&);
[[nodiscard]] Vector<ComponentValue> substitute_arbitrary_substitution_functions(DOM::AbstractElement&, GuardedSubstitutionContexts&, Vector<ComponentValue> const&, Optional<SubstitutionContext> = {});
using ArbitrarySubstitutionFunctionArguments = Vector<Vector<ComponentValue>>;
[[nodiscard]] Optional<ArbitrarySubstitutionFunctionArguments> parse_according_to_argument_grammar(ArbitrarySubstitutionFunction, Vector<ComponentValue> const&);
[[nodiscard]] Vector<ComponentValue> replace_an_arbitrary_substitution_function(DOM::AbstractElement&, GuardedSubstitutionContexts&, ArbitrarySubstitutionFunction, ArbitrarySubstitutionFunctionArguments const&);
}

7
Libraries/LibWeb/CSS/Parser/Parser.h
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@ -134,7 +134,7 @@ public:
Vector<ComponentValue> parse_as_list_of_component_values();
static NonnullRefPtr<CSSStyleValue const> resolve_unresolved_style_value(ParsingParams const&, DOM::Element&, Optional<PseudoElement>, PropertyIDOrCustomPropertyName, UnresolvedStyleValue const&);
static NonnullRefPtr<CSSStyleValue const> resolve_unresolved_style_value(ParsingParams const&, DOM::Element&, Optional<PseudoElement>, PropertyIDOrCustomPropertyName, UnresolvedStyleValue const&, Optional<GuardedSubstitutionContexts&> = {});
[[nodiscard]] LengthOrCalculated parse_as_sizes_attribute(DOM::Element const& element, HTML::HTMLImageElement const* img = nullptr);
@ -513,10 +513,7 @@ private:
OwnPtr<BooleanExpression> parse_supports_feature(TokenStream<ComponentValue>&);
NonnullRefPtr<CSSStyleValue const> resolve_unresolved_style_value(DOM::Element&, Optional<PseudoElement>, PropertyIDOrCustomPropertyName, UnresolvedStyleValue const&);
bool expand_variables(DOM::Element&, Optional<PseudoElement>, FlyString const& property_name, HashMap<FlyString, NonnullRefPtr<PropertyDependencyNode>>& dependencies, TokenStream<ComponentValue>& source, Vector<ComponentValue>& dest);
bool expand_unresolved_values(DOM::Element&, FlyString const& property_name, TokenStream<ComponentValue>& source, Vector<ComponentValue>& dest);
bool substitute_attr_function(DOM::Element& element, FlyString const& property_name, Function const& attr_function, Vector<ComponentValue>& dest);
NonnullRefPtr<CSSStyleValue const> resolve_unresolved_style_value(DOM::AbstractElement&, GuardedSubstitutionContexts&, PropertyIDOrCustomPropertyName, UnresolvedStyleValue const&);
static bool has_ignored_vendor_prefix(StringView);

352
Libraries/LibWeb/CSS/Parser/ValueParsing.cpp
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@ -19,8 +19,8 @@
#include <AK/StringConversions.h>
#include <AK/TemporaryChange.h>
#include <LibWeb/CSS/FontFace.h>
#include <LibWeb/CSS/Parser/ArbitrarySubstitutionFunctions.h>
#include <LibWeb/CSS/Parser/Parser.h>
#include <LibWeb/CSS/PropertyName.h>
#include <LibWeb/CSS/StyleValues/AngleStyleValue.h>
#include <LibWeb/CSS/StyleValues/BackgroundRepeatStyleValue.h>
#include <LibWeb/CSS/StyleValues/BackgroundSizeStyleValue.h>
@ -4273,342 +4273,56 @@ RefPtr<FontSourceStyleValue const> Parser::parse_font_source_value(TokenStream<C
return FontSourceStyleValue::create(url.release_value(), move(format), move(tech));
}
NonnullRefPtr<CSSStyleValue const> Parser::resolve_unresolved_style_value(ParsingParams const& context, DOM::Element& element, Optional<PseudoElement> pseudo_element, PropertyIDOrCustomPropertyName property, UnresolvedStyleValue const& unresolved)
NonnullRefPtr<CSSStyleValue const> Parser::resolve_unresolved_style_value(ParsingParams const& context, DOM::Element& element, Optional<PseudoElement> pseudo_element, PropertyIDOrCustomPropertyName property, UnresolvedStyleValue const& unresolved, Optional<GuardedSubstitutionContexts&> existing_guarded_contexts)
{
// Unresolved always contains a var() or attr(), unless it is a custom property's value, in which case we shouldn't be trying
// to produce a different CSSStyleValue from it.
VERIFY(unresolved.contains_arbitrary_substitution_function());
DOM::AbstractElement abstract_element { element, pseudo_element };
auto parser = Parser::create(context, ""sv);
return parser.resolve_unresolved_style_value(element, pseudo_element, property, unresolved);
if (existing_guarded_contexts.has_value())
return parser.resolve_unresolved_style_value(abstract_element, existing_guarded_contexts.value(), property, unresolved);
GuardedSubstitutionContexts guarded_contexts;
return parser.resolve_unresolved_style_value(abstract_element, guarded_contexts, property, unresolved);
}
class PropertyDependencyNode : public RefCounted<PropertyDependencyNode> {
public:
static NonnullRefPtr<PropertyDependencyNode> create(FlyString name)
{
return adopt_ref(*new PropertyDependencyNode(move(name)));
}
void add_child(NonnullRefPtr<PropertyDependencyNode> new_child)
{
for (auto const& child : m_children) {
if (child->m_name == new_child->m_name)
return;
}
// We detect self-reference already.
VERIFY(new_child->m_name != m_name);
m_children.append(move(new_child));
}
bool has_cycles()
{
if (m_marked)
return true;
TemporaryChange change { m_marked, true };
for (auto& child : m_children) {
if (child->has_cycles())
return true;
}
return false;
}
private:
explicit PropertyDependencyNode(FlyString name)
: m_name(move(name))
{
}
FlyString m_name;
Vector<NonnullRefPtr<PropertyDependencyNode>> m_children;
bool m_marked { false };
};
NonnullRefPtr<CSSStyleValue const> Parser::resolve_unresolved_style_value(DOM::Element& element, Optional<PseudoElement> pseudo_element, PropertyIDOrCustomPropertyName property, UnresolvedStyleValue const& unresolved)
// https://drafts.csswg.org/css-values-5/#property-replacement
NonnullRefPtr<CSSStyleValue const> Parser::resolve_unresolved_style_value(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, PropertyIDOrCustomPropertyName property, UnresolvedStyleValue const& unresolved)
{
TokenStream unresolved_values_without_variables_expanded { unresolved.values() };
Vector<ComponentValue> values_with_variables_expanded;
// AD-HOC: Report that we might rely on custom properties.
// FIXME: This over-invalidates. Find a way of invalidating only when we need to - specifically, when var() is used.
element.element().set_style_uses_css_custom_properties(true);
// To replace substitution functions in a property prop:
auto const& property_name = property.visit(
[](PropertyID const& property_id) { return string_from_property_id(property_id); },
[](FlyString const& name) { return name; });
auto const& property_id = property.visit(
[](PropertyID const& property_id) { return property_id; },
[](FlyString const&) { return PropertyID::Custom; });
HashMap<FlyString, NonnullRefPtr<PropertyDependencyNode>> dependencies;
ScopeGuard mark_element_if_uses_custom_properties = [&] {
for (auto const& name : dependencies.keys()) {
if (is_a_custom_property_name_string(name)) {
element.set_style_uses_css_custom_properties(true);
return;
}
}
};
if (!expand_variables(element, pseudo_element, property_name, dependencies, unresolved_values_without_variables_expanded, values_with_variables_expanded))
// 1. Substitute arbitrary substitution functions in prop’s value, given «"property", prop’s name» as the
// substitution context. Let result be the returned component value sequence.
auto result = substitute_arbitrary_substitution_functions(element, guarded_contexts, unresolved.values(), SubstitutionContext { SubstitutionContext::DependencyType::Property, property_name.to_string() });
// 2. If result contains the guaranteed-invalid value, prop is invalid at computed-value time; return.
if (contains_guaranteed_invalid_value(result))
return GuaranteedInvalidStyleValue::create();
TokenStream unresolved_values_with_variables_expanded { values_with_variables_expanded };
Vector<ComponentValue> expanded_values;
if (!expand_unresolved_values(element, property_name, unresolved_values_with_variables_expanded, expanded_values))
// 3. Parse result according to prop’s grammar. If this returns failure, prop is invalid at computed-value time; return.
// NB: Custom properties have no grammar as such, so we skip this step for them.
// FIXME: Parse according to @property syntax once we support that.
if (property_id == PropertyID::Custom)
return UnresolvedStyleValue::create(move(result), false, {});
auto expanded_value_tokens = TokenStream { result };
auto parsed_value = parse_css_value(property_id, expanded_value_tokens);
if (parsed_value.is_error())
return GuaranteedInvalidStyleValue::create();
return property.visit(
[&](PropertyID const& property_id) -> NonnullRefPtr<CSSStyleValue const> {
auto expanded_value_tokens = TokenStream { expanded_values };
if (auto parsed_value = parse_css_value(property_id, expanded_value_tokens); !parsed_value.is_error())
return parsed_value.release_value();
return GuaranteedInvalidStyleValue::create();
},
[&](FlyString const&) -> NonnullRefPtr<CSSStyleValue const> {
return UnresolvedStyleValue::create(move(expanded_values), false, {});
});
}
static RefPtr<CSSStyleValue const> get_custom_property(DOM::Element const& element, Optional<CSS::PseudoElement> pseudo_element, FlyString const& custom_property_name)
{
if (pseudo_element.has_value()) {
if (auto it = element.custom_properties(pseudo_element).find(custom_property_name); it != element.custom_properties(pseudo_element).end())
return it->value.value;
}
for (auto const* current_element = &element; current_element; current_element = current_element->parent_or_shadow_host_element()) {
if (auto it = current_element->custom_properties({}).find(custom_property_name); it != current_element->custom_properties({}).end())
return it->value.value;
}
return nullptr;
}
bool Parser::expand_variables(DOM::Element& element, Optional<PseudoElement> pseudo_element, FlyString const& property_name, HashMap<FlyString, NonnullRefPtr<PropertyDependencyNode>>& dependencies, TokenStream<ComponentValue>& source, Vector<ComponentValue>& dest)
{
// Arbitrary large value chosen to avoid the billion-laughs attack.
// https://www.w3.org/TR/css-variables-1/#long-variables
size_t const MAX_VALUE_COUNT = 16384;
if (source.remaining_token_count() + dest.size() > MAX_VALUE_COUNT) {
dbgln("Stopped expanding CSS variables: maximum length reached.");
return false;
}
auto get_dependency_node = [&](FlyString const& name) -> NonnullRefPtr<PropertyDependencyNode> {
if (auto existing = dependencies.get(name); existing.has_value())
return *existing.value();
auto new_node = PropertyDependencyNode::create(name);
dependencies.set(name, new_node);
return new_node;
};
while (source.has_next_token()) {
auto const& value = source.consume_a_token();
if (value.is_block()) {
auto const& source_block = value.block();
Vector<ComponentValue> block_values;
TokenStream source_block_contents { source_block.value };
if (!expand_variables(element, pseudo_element, property_name, dependencies, source_block_contents, block_values))
return false;
dest.empend(SimpleBlock { source_block.token, move(block_values) });
continue;
}
if (!value.is_function()) {
dest.empend(value.token());
continue;
}
if (!value.function().name.equals_ignoring_ascii_case("var"sv)) {
auto const& source_function = value.function();
Vector<ComponentValue> function_values;
TokenStream source_function_contents { source_function.value };
if (!expand_variables(element, pseudo_element, property_name, dependencies, source_function_contents, function_values))
return false;
dest.empend(Function { source_function.name, move(function_values) });
continue;
}
TokenStream var_contents { value.function().value };
var_contents.discard_whitespace();
if (!var_contents.has_next_token())
return false;
auto const& custom_property_name_token = var_contents.consume_a_token();
if (!custom_property_name_token.is(Token::Type::Ident))
return false;
auto custom_property_name = custom_property_name_token.token().ident();
if (!custom_property_name.bytes_as_string_view().starts_with("--"sv))
return false;
// Detect dependency cycles. https://www.w3.org/TR/css-variables-1/#cycles
// We do not do this by the spec, since we are not keeping a graph of var dependencies around,
// but rebuilding it every time.
if (custom_property_name == property_name)
return false;
auto parent = get_dependency_node(property_name);
auto child = get_dependency_node(custom_property_name);
parent->add_child(child);
if (parent->has_cycles())
return false;
if (auto custom_property_value = get_custom_property(element, pseudo_element, custom_property_name);
custom_property_value && custom_property_value->is_unresolved()) {
// FIXME: We should properly cascade here instead of doing a basic fallback for CSS-wide keywords.
TokenStream custom_property_tokens { custom_property_value->as_unresolved().values() };
auto dest_size_before = dest.size();
if (!expand_variables(element, pseudo_element, custom_property_name, dependencies, custom_property_tokens, dest))
return false;
// If the size of dest has increased, then the custom property is not the initial guaranteed-invalid value.
// If it hasn't increased, then it is the initial guaranteed-invalid value, and thus we should move on to the fallback value.
if (dest_size_before < dest.size())
continue;
dbgln_if(CSS_PARSER_DEBUG, "CSSParser: Expanding custom property '{}' did not return any tokens, treating it as invalid and moving on to the fallback value.", custom_property_name);
}
// Use the provided fallback value, if any.
var_contents.discard_whitespace();
if (var_contents.has_next_token()) {
auto const& comma_token = var_contents.consume_a_token();
if (!comma_token.is(Token::Type::Comma))
return false;
var_contents.discard_whitespace();
if (!expand_variables(element, pseudo_element, property_name, dependencies, var_contents, dest))
return false;
}
}
return true;
}
bool Parser::expand_unresolved_values(DOM::Element& element, FlyString const& property_name, TokenStream<ComponentValue>& source, Vector<ComponentValue>& dest)
{
while (source.has_next_token()) {
auto const& value = source.consume_a_token();
if (value.is_function()) {
if (value.function().name.equals_ignoring_ascii_case("attr"sv)) {
if (!substitute_attr_function(element, property_name, value.function(), dest))
return false;
continue;
}
auto const& source_function = value.function();
Vector<ComponentValue> function_values;
TokenStream source_function_contents { source_function.value };
if (!expand_unresolved_values(element, property_name, source_function_contents, function_values))
return false;
dest.empend(Function { source_function.name, move(function_values) });
continue;
}
if (value.is_block()) {
auto const& source_block = value.block();
TokenStream source_block_values { source_block.value };
Vector<ComponentValue> block_values;
if (!expand_unresolved_values(element, property_name, source_block_values, block_values))
return false;
dest.empend(SimpleBlock { source_block.token, move(block_values) });
continue;
}
dest.empend(value.token());
}
return true;
}
// https://drafts.csswg.org/css-values-5/#attr-substitution
bool Parser::substitute_attr_function(DOM::Element& element, FlyString const& property_name, Function const& attr_function, Vector<ComponentValue>& dest)
{
// attr() = attr( <attr-name> <attr-type>? , <declaration-value>?)
// <attr-name> = [ <ident-token>? '|' ]? <ident-token>
// <attr-type> = type( <syntax> ) | raw-string | <attr-unit>
// The <attr-unit> production matches any identifier that is an ASCII case-insensitive match for the name of a CSS dimension unit, such as px, or the <delim-token> %.
TokenStream attr_contents { attr_function.value };
attr_contents.discard_whitespace();
if (!attr_contents.has_next_token())
return false;
// - Attribute name
// FIXME: Support optional attribute namespace
if (!attr_contents.next_token().is(Token::Type::Ident))
return false;
auto attribute_name = attr_contents.consume_a_token().token().ident();
attr_contents.discard_whitespace();
// - Attribute type (optional)
auto attribute_type = "raw-string"_fly_string;
if (attr_contents.next_token().is(Token::Type::Ident)) {
attribute_type = attr_contents.consume_a_token().token().ident();
attr_contents.discard_whitespace();
}
// - Comma, then fallback values (optional)
bool has_fallback_values = false;
if (attr_contents.has_next_token()) {
if (!attr_contents.next_token().is(Token::Type::Comma))
return false;
(void)attr_contents.consume_a_token(); // Comma
has_fallback_values = true;
}
// Then, run the substitution algorithm:
// 1. If the attr() function has a substitution value, replace the attr() function by the substitution value.
// https://drafts.csswg.org/css-values-5/#attr-types
if (element.has_attribute(attribute_name)) {
auto parse_string_as_component_value = [this](String const& string) {
auto tokens = Tokenizer::tokenize(string, "utf-8"sv);
TokenStream stream { tokens };
return parse_a_component_value(stream);
};
auto attribute_value = element.get_attribute_value(attribute_name);
if (attribute_type.equals_ignoring_ascii_case("raw-string"_fly_string)) {
// The substitution value is a CSS string, whose value is the literal value of the attribute.
// (No CSS parsing or "cleanup" of the value is performed.)
// No value triggers fallback.
dest.empend(Token::create_string(attribute_value));
return true;
} else {
// Dimension units
// Parse a component value from the attribute’s value.
// If the result is a <number-token>, the substitution value is a dimension with the result’s value, and the given unit.
// Otherwise, there is no substitution value.
auto component_value = parse_string_as_component_value(attribute_value);
if (component_value.has_value() && component_value->is(Token::Type::Number)) {
if (attribute_value == "%"sv) {
dest.empend(Token::create_dimension(component_value->token().number_value(), attribute_type));
return true;
} else if (auto angle_unit = Angle::unit_from_name(attribute_type); angle_unit.has_value()) {
dest.empend(Token::create_dimension(component_value->token().number_value(), attribute_type));
return true;
} else if (auto flex_unit = Flex::unit_from_name(attribute_type); flex_unit.has_value()) {
dest.empend(Token::create_dimension(component_value->token().number_value(), attribute_type));
return true;
} else if (auto frequency_unit = Frequency::unit_from_name(attribute_type); frequency_unit.has_value()) {
dest.empend(Token::create_dimension(component_value->token().number_value(), attribute_type));
return true;
} else if (auto length_unit = Length::unit_from_name(attribute_type); length_unit.has_value()) {
dest.empend(Token::create_dimension(component_value->token().number_value(), attribute_type));
return true;
} else if (auto time_unit = Time::unit_from_name(attribute_type); time_unit.has_value()) {
dest.empend(Token::create_dimension(component_value->token().number_value(), attribute_type));
return true;
} else {
// Not a dimension unit.
return false;
}
}
}
}
// 2. Otherwise, if the attr() function has a fallback value as its last argument, replace the attr() function by the fallback value.
// If there are any var() or attr() references in the fallback, substitute them as well.
if (has_fallback_values)
return expand_unresolved_values(element, property_name, attr_contents, dest);
if (attribute_type.equals_ignoring_ascii_case("raw-string"_fly_string)) {
// If the <attr-type> argument is string, defaults to the empty string if omitted
dest.empend(Token::create_string({}));
return true;
}
// 3. Otherwise, the property containing the attr() function is invalid at computed-value time.
return false;
// 4. Otherwise, replace prop’s value with the parsed result.
return parsed_value.release_value();
}
}

5
Libraries/LibWeb/CSS/StyleComputer.cpp
View file

@ -40,6 +40,7 @@
#include <LibWeb/CSS/Fetch.h>
#include <LibWeb/CSS/Interpolation.h>
#include <LibWeb/CSS/InvalidationSet.h>
#include <LibWeb/CSS/Parser/ArbitrarySubstitutionFunctions.h>
#include <LibWeb/CSS/Parser/Parser.h>
#include <LibWeb/CSS/SelectorEngine.h>
#include <LibWeb/CSS/StyleComputer.h>
@ -3095,7 +3096,7 @@ void StyleComputer::unload_fonts_from_sheet(CSSStyleSheet& sheet)
}
}
NonnullRefPtr<CSSStyleValue const> StyleComputer::compute_value_of_custom_property(DOM::AbstractElement abstract_element, FlyString const& name)
NonnullRefPtr<CSSStyleValue const> StyleComputer::compute_value_of_custom_property(DOM::AbstractElement abstract_element, FlyString const& name, Optional<Parser::GuardedSubstitutionContexts&> guarded_contexts)
{
// https://drafts.csswg.org/css-variables/#propdef-
// The computed value of a custom property is its specified value with any arbitrary-substitution functions replaced.
@ -3129,7 +3130,7 @@ NonnullRefPtr<CSSStyleValue const> StyleComputer::compute_value_of_custom_proper
return value.release_nonnull();
auto& unresolved = value->as_unresolved();
return Parser::Parser::resolve_unresolved_style_value(Parser::ParsingParams {}, abstract_element.element(), abstract_element.pseudo_element(), name, unresolved);
return Parser::Parser::resolve_unresolved_style_value(Parser::ParsingParams {}, abstract_element.element(), abstract_element.pseudo_element(), name, unresolved, guarded_contexts);
}
void StyleComputer::compute_custom_properties(ComputedProperties&, DOM::AbstractElement abstract_element) const

2
Libraries/LibWeb/CSS/StyleComputer.h
View file

@ -193,7 +193,7 @@ public:
[[nodiscard]] inline bool should_reject_with_ancestor_filter(Selector const&) const;
static NonnullRefPtr<CSSStyleValue const> compute_value_of_custom_property(DOM::AbstractElement, FlyString const& custom_property);
static NonnullRefPtr<CSSStyleValue const> compute_value_of_custom_property(DOM::AbstractElement, FlyString const& custom_property, Optional<Parser::GuardedSubstitutionContexts&> = {});
private:
enum class ComputeStyleMode {