ladybird/Documentation/CSSGeneratedFiles.md

CSS Generated Files

We generate a significant amount of CSS-related code, taking in one or more .json files in Libraries/LibWeb/CSS and producing C++ code from them, located in Build/<build-preset>/Lagom/Libraries/LibWeb/CSS/. It's likely that you'll need to work with these if you add or modify a CSS property or its values.

The generators are found in Meta/Lagom/Tools/CodeGenerators/LibWeb. They are run automatically as part of the build, and most of the time you can ignore them.

Properties.json

Each CSS property has an entry here, which describes what values it accepts, whether it's inherited, and similar data. This generates PropertyID.h, PropertyID.cpp, GeneratedCSSStyleProperties.h, GeneratedCSSStyleProperties.cpp and GeneratedCSSStyleProperties.idl. Most of this data is found in the information box for that property in the relevant CSS spec.

The file is organized as a single JSON object, with keys being property names, and the values being the data for that property. Each property will have some set of these fields on it:

(Note that required fields are not required on properties with legacy-alias-for or logical-alias-for set.)

Field	Required	Default	Description	Generated functions
affects-layout	No	true	Boolean. Whether changing this property will invalidate the element's layout.	bool property_affects_layout(PropertyID)
affects-stacking-context	No	false	Boolean. Whether this property can cause a new stacking context for the element.	bool property_affects_stacking_context(PropertyID)
animation-type	Yes		String. How the property should be animated. Defined by the spec. See below.	AnimationType animation_type_from_longhand_property(PropertyID)
inherited	Yes		Boolean. Whether the property is inherited by its child elements.	bool is_inherited_property(PropertyID)
initial	Yes		String. The property's initial value if it is not specified.	NonnullRefPtr<StyleValue const> property_initial_value(PropertyID)
legacy-alias-for	No	Nothing	String. The name of a property this is an alias for. See below.
logical-alias-for	No	Nothing	An object. See below.	bool property_is_logical_alias(PropertyID); PropertyID map_logical_alias_to_physical_property(PropertyID, LogicalAliasMappingContext const&)
longhands	No	[]	Array of strings. If this is a shorthand, these are the property names that it expands out into.	Vector<PropertyID> longhands_for_shorthand(PropertyID) Vector<PropertyID> expanded_longhands_for_shorthand(PropertyID) Vector<PropertyID> shorthands_for_longhand(PropertyID)
max-values	No	1	Integer. How many values can be parsed for this property. eg, margin can have up to 4 values.	size_t property_maximum_value_count(PropertyID)
multiplicity	No	"single"	String. Category for whether this property is a single value or a list of values. See below.	bool property_is_single_valued(PropertyID) bool property_is_list_valued(PropertyID)
percentages-resolve-to	No	Nothing	String. What type percentages get resolved to. eg, for width percentages are resolved to length values.	Optional<ValueType> property_resolves_percentages_relative_to(PropertyID)
positional-value-list-shorthand	No	false	Boolean. Whether this property is a "positional value list shorthand". See below.	bool property_is_positional_value_list_shorthand(PropertyID)
quirks	No	[]	Array of strings. Some properties have special behavior in "quirks mode", which are listed here. See below.	bool property_has_quirk(PropertyID, Quirk)
valid-identifiers	No	[]	Array of strings. Which keywords the property accepts. See below.	bool property_accepts_keyword(PropertyID, Keyword) Optional<Keyword> resolve_legacy_value_alias(PropertyID, Keyword)
valid-types	No	[]	Array of strings. Which value types the property accepts. See below.	bool property_accepts_type(PropertyID, ValueType)
needs-layout-for-getcomputedstyle	No	false	Boolean. Whether this property requires up-to-date layout before it could be queried by getComputedStyle()	bool property_needs_layout_for_getcomputedstyle(PropertyID)

animation-type

The Web Animations spec defines the valid values here:

Spec term	JSON value
not animatable	none
discrete	discrete
by computed value	by-computed-value
repeatable list	repeatable-list
(See prose)	custom

legacy-alias-for

(Not to be confused with logical-alias-for below.)

Legacy name aliases are properties whose spec names have changed, but the syntax has not, so setting the old one is defined as setting the new one directly. For example, font-stretch was renamed to font-width, so font-stretch is now a legacy name alias for font-width.

logical-alias-for

(Not to be confused with legacy-alias-for above.)

Logical aliases are properties like margin-block-start, which may assign a value to one of several other properties (margin-top, margin-bottom, margin-left, or margin-right) depending on the element they are applied to.

logical-alias-for should be an object with two fields, both of which are required:

Field	Description
group	String. Name of the logical property group this is associated with. (See LogicalPropertyGroups.json.)
mapping	String. How this relates to the group. eg, if it's the block end value, block-end.

multiplicity

The three possible values are "single", "list", and "coordinating-list".

Most properties represent a single "thing", possibly made of multiple parts. For example, border takes a color, style and thickness, but it's still a single border. Others are a list: background takes multiple layers, separated by commas; counter-increment doesn't have commas but also is a list of counters that are incremented.

Lists can also be coordinating list properties, in which case they are marked as coordinating-list instead of list.

positional-value-list-shorthand

Some shorthand properties work differently to normal in that mapping of provided values to longhands isn't necessarily 1-to-1 and instead depends on the number of values provided, for example margin, border-width, gap, etc.

These properties have distinct behaviors in how they are parsed and serialized, having them marked allows us to implement this behavior in a generic way.

quirks

The Quirks spec defines these.

Spec term	JSON value
The hashless hex color quirk	hashless-hex-color
The unitless length quirk	unitless-length

valid-identifiers

A list of CSS keyword names, that the property accepts. Consider defining an enum instead and putting its name in the valid-types array, especially if the spec provides a name to a set of such keywords.

Some properties have legacy value aliases, where one keyword is parsed as another. These are supported as "foo>bar", to make foo an alias for bar.

valid-types

The valid-types array lists the names of CSS value types, as defined in the latest CSS Values and Units spec, without the <> around them. For numeric types, we use the bracketed range notation, for example width can take any non-negative length, so it has "length [0,∞]" in its valid-types array. For <custom-ident>s, the excluded identifiers are placed within ![], for example "custom-ident ![all,none]".

LogicalPropertyGroups.json

A set of matching CSS properties can be grouped together in what's called a "logical property group". For example, all of the margin-* properties are in the margin group.

This data is used to map logical properties (such as margin-block-start) into their physical counterparts (like margin-top) at runtime, depending on the writing direction. We don't generate any code directly from this file. Instead, it's used as part of generating the PropertyID code.

The file is a single object where the keys are the names of the logical property groups, and their values are objects mapping physical dimensions, sides, or corners to the relevant property. Which keys are there depends on the group - for example the size group has width and height.

Descriptors.json

Descriptors are basically properties, but for at-rules instead of style. The overall structure is a JSON object, with keys being at-rule names and the values being data about those at-rules. The main part is the data about the descriptors that the at-rule can have.

The generated code provides:

• An AtRuleID enum, mostly used as a parameter for parsing descriptors, as multiple at-rules may have descriptors with the same name.
• FlyString to_string(AtRuleID), mostly for debug logging.
• A DescriptorID enum, listing every descriptor.
• Optional<DescriptorID> descriptor_id_from_string(AtRuleID, StringView) for getting a DescriptorID from a string, if it exists in that at-rule.
• FlyString to_string(DescriptorID) for serializing descriptor names.
• bool at_rule_supports_descriptor(AtRuleID, DescriptorID) to query if the given at-rule allows the descriptor.
• RefPtr<StyleValue const> descriptor_initial_value(AtRuleID, DescriptorID) for getting a descriptor's initial value.
• DescriptorMetadata get_descriptor_metadata(AtRuleID, DescriptorID) returns data used for parsing the descriptor.

At-rule fields

Each at-rule object has the following fields. Both are required.

Field	Description
spec	String. URL to the spec that defines this at-rule.
descriptors	Object, with keys being descriptor names and values being objects of their properties. See below.

Descriptor fields

Each descriptor object can have the following fields:

Field	Required	Description
initial	No	String. The descriptor's initial value if none is provided.
legacy-alias-for	No	String. The name of a different descriptor that this is an alias for.
syntax	Yes	Array of strings. Each string is one option, taken from the spec.
FIXME or NOTE	No	Strings, for when you want to leave a note.

Keywords.json

This is a single JSON array of strings, each of which is a CSS keyword, for example auto, none, medium, or currentcolor. This generates Keyword.h and Keyword.cpp. All keyword values used by any property or media-feature need to be defined here.

The generated code provides:

• A Keyword enum as used by KeywordStyleValue
• Optional<Keyword> keyword_from_string(StringView) to attempt to convert a string into a Keyword
• StringView string_from_keyword(Keyword) to convert a Keyword back into a string
• bool is_css_wide_keyword(StringView) which returns whether the string is one of the special "CSS-wide keywords"

Enums.json

This is a single JSON object, with enum names as keys and the values being arrays of keyword names. This generates Enums.h and Enums.cpp.

We often want to define an enum that's a set of a few keywords. Enums.json allows you to generate these enums automatically, along with functions to convert them to and from a Keyword, or convert them to a string. These enums also can be used in property definitions in Properties.json by putting their name in the valid-types array. This helps reduce repetition, for example the border-*-style properties all accept the same set of keywords, so they are implemented as a line-style enum.

The generated code provides these for each enum, using "foo" as an example:

• A Foo enum for its values
• Optional<Foo> keyword_to_foo(Keyword) to convert a Keyword to a Foo
• Keyword to_keyword(Foo) to convert the Foo back to a Keyword
• StringView to_string(Foo) to convert the Foo directly to a string

PseudoClasses.json

This is a single JSON object, with selector pseudo-class names as keys and the values being objects with fields for the pseudo-class. This generates PseudoClass.h and PseudoClass.cpp.

Each entry has a single required property, argument, which is a string containing the grammar for the pseudo-class's function parameters - for identifier-style pseudo-classes it is left blank. The grammar is taken directly from the spec.

The generated code provides:

• A PseudoClass enum listing every pseudo-class name
• Optional<PseudoClass> pseudo_class_from_string(StringView) to parse a string as a PseudoClass name
• StringView pseudo_class_name(PseudoClass) to convert a PseudoClass back into a string
• The PseudoClassMetadata struct which holds a representation of the data from the JSON file
• PseudoClassMetadata pseudo_class_metadata(PseudoClass) to retrieve that data

PseudoElements.json

This is a single JSON object, with pseudo-element names as keys and the values being objects with fields for the pseudo-element. This generated PseudoElement.h and PseudoElement.cpp.

Each entry has the following properties:

Field	Required	Default	Description
alias-for	No	Nothing	Use to specify that this should be treated as an alias for the named pseudo-element.
function-syntax	No	Nothing	Syntax for the function arguments if this is a function-type pseudo-element. Copied directly from the spec.
is-allowed-in-has	No	false	Whether this is a :has-allowed pseudo-element.
is-pseudo-root	No	false	Whether this is a pseudo-element root.
property-whitelist	No	Nothing	Some pseudo-elements only permit certain properties. If so, name them in an array here. Some special values are allowed here for categories of properties - see below.
spec	No	Nothing	Link to the spec definition, for reference. Not used in generated code.
type	No	"identifier"	What type of pseudo-element is this. Either "identifier", "function", or "both".

The generated code provides:

• A PseudoElement enum listing every pseudo-element name
• Optional<PseudoElement> pseudo_element_from_string(StringView) to parse a string as a PseudoElement name
• Optional<PseudoElement> aliased_pseudo_element_from_string(StringView) is similar, but returns the PseudoElement this name is an alias for
• StringView pseudo_element_name(PseudoElement) to convert a PseudoElement back into a string
• bool is_has_allowed_pseudo_element(PseudoElement) returns whether the pseudo-element is valid inside :has()
• bool is_pseudo_element_root(PseudoElement) returns whether the pseudo-element is a pseudo-element root
• bool pseudo_element_supports_property(PseudoElement, PropertyID) returns whether the property can be applied to this pseudo-element

property-whitelist

This is an array of strings. Properties can be named directly ("color"), or categories of properties with a leading # ("#font-properties"), as the specs often says a group is allowed instead of listing the properties exactly. Any properties we don't support yet can be prefixed with "FIXME:" and will be ignored.

The following categories are supported:

• #background-properties: background and its longhands
• #border-properties: border, border-radius, and their longhands
• #custom-properties: Custom properties, AKA CSS variables
• #font-properties: font, its longhands, and other font-* properties
• #inline-layout-properties: Properties defined in CSS Inline
• #inline-typesetting-properties: Properties defined in CSS Text
• #margin-properties: margin and its longhands
• #padding-properties: padding and its longhands
• #text-decoration-properties: text-decoration and its longhands

MediaFeatures.json

This is a single JSON object, with media-feature names as keys and the values being objects with fields for the media-feature. This generates MediaFeatureID.h and MediaFeatureID.cpp.

A <media-feature> is a value that a media query can inspect. They are listed in the @media descriptor table in the latest Media Queries spec.

The definitions here are like a simplified version of the Properties.json definitions.

Field	Description
type	String. How the media-feature is evaluated, either discrete or range.
values	Array of strings. These are directly taken from the spec, with keywords as they are, and <> around type names. Types may be <boolean>, <integer>, <length>, <ratio>, or <resolution>.
false-keywords	Array of strings. These are any keywords that should be considered false when the media feature is evaluated as @media (foo). Generally this will be a single value, such as "none".

The generated code provides:

• A MediaFeatureValueType enum listing the possible value types
• A MediaFeatureID enum, listing each media-feature
• Optional<MediaFeatureID> media_feature_id_from_string(StringView) to convert a string to a MediaFeatureID
• StringView string_from_media_feature_id(MediaFeatureID) to convert a MediaFeatureID back to a string
• bool media_feature_type_is_range(MediaFeatureID) returns whether the media feature is a range type, as opposed to a discrete type
• bool media_feature_accepts_type(MediaFeatureID, MediaFeatureValueType) returns whether the media feature will accept values of this type
• bool media_feature_accepts_keyword(MediaFeatureID, Keyword) returns whether the media feature accepts this keyword
• bool media_feature_keyword_is_falsey(MediaFeatureID, Keyword) returns whether the given keyword is considered false when the media-feature is evaluated in a boolean context. (Like @media (foo))

MathFunctions.json

This is a single JSON object, describing each CSS math function, with the keys being the function name and the values being objects describing that function's properties. This generates MathFunctions.h and MathFunctions.cpp.

Each entry has two properties:

Field	Description
parameter-validation	Optional string. Either "same" or "consistent", depending on whether the spec says the input calculations should be the same type or consistent types. Defaults to "same". Ignore this if there is only one parameter.
parameters	An array of parameter definition objects, see below.

Parameter definitions have the following properties:

Field	Description
name	String. Name of the parameter, as given in the spec.
type	String. Accepted types for the parameter, as a single string, separated by |.
required	Boolean. Whether this parameter is required.

The generated code provides:

• A MathFunction enum listing the math functions
• The implementation of the CSS Parser's parse_math_function() method

TransformFunctions.json

This is a single JSON object, describing each CSS transform function, with the keys being the function name and the values being objects describing that function's properties. This generates TransformFunctions.h and TransformFunctions.cpp.

Each entry currently has a single property, parameters, which is an array of parameter definition objects. Parameter definitions have the following properties:

Field	Description
type	String. Accepted type for the parameter.
required	Boolean. Whether this parameter is required.

The generated code provides:

• A TransformFunction enum listing the transform functions
• Optional<TransformFunction> transform_function_from_string(StringView) to parse a string as a TransformFunction
• StringView to_string(TransformFunction) to convert a TransformFunction back to a string
• TransformFunctionMetadata transform_function_metadata(TransformFunction) to obtain metadata about the transform function, such as its parameter list

EnvironmentVariables.json

This is a single JSON object, describing each CSS environment variable, with the keys being the environment variable names, and the values being objects describing the variable's properties. This generates EnvironmentVariable.h and EnvironmentVariable.cpp.

Each entry has 3 properties, all taken from the spec:

Field	Description
spec	String. URL to the spec definition for this environment variable.
type	String. CSS value type of the variable, eg <length>.
dimensions	Integer. Number of dimensions for the variable, or 0 for scalars.

The generated code provides:

• An EnvironmentVariable enum listing the environment variables
• Optional<EnvironmentVariable> environment_variable_from_string(StringView) to parse a string as an EnvironmentVariable
• StringView to_string(EnvironmentVariable) to convert the EnvironmentVariable back to a string
• ValueType environment_variable_type(EnvironmentVariable) to get the variable's value type
• u32 environment_variable_dimension_count(EnvironmentVariable) to get its dimension count

Units.json

This is a JSON object with the keys being dimension type names, and the values being objects. Those objects' keys are unit names, and their values are data about each unit. It generates Units.h and Units.cpp.

Each unit has the following properties:

Field	Description
is-canonical-unit	Boolean, default false. Each dimension has one canonical unit.
number-of-canonical-unit	Number. How many of the canonical units 1 of this is equivalent to. Ignore this for relative units, and the canonical unit itself.
is-relative-to	String. Some length units are relative to the font or viewport. Set this to "font" or "viewport" for those.

The generated code provides:

• A DimensionType enum, listing each type of dimension that has units defined.
• Optional<DimensionType> dimension_for_unit(StringView) for querying which dimension a unit applies to, if any.
• A FooUnit enum for each dimension "foo", which lists all the units of that dimension.
• For each of those...
  • constexpr FooUnit canonical_foo_unit() which is the canonical unit for that type.
  • Optional<FooUnit> string_to_foo_unit(StringView) for parsing a unit from a string.
  • StringView to_string(FooUnit) for serializing those units.
  • bool units_are_compatible(FooUnit, FooUnit) which returns whether these are compatible - basically whether you can convert from one to the other.
  • double ratio_between_units(FooUnit, FooUnit) to get a multiplier for converting the first unit into the second.
• bool is_absolute(LengthUnit), bool is_font_relative(LengthUnit), bool is_viewport_relative(LengthUnit), and bool is_relative(LengthUnit) for checking the category of length units.