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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<CSSStyleValue> 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 Array of strings. The name of a property this is an alias for. See below.
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)
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)
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)
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. Consider defining an enum instead and putting its name in the valid-types array. bool property_accepts_keyword(PropertyID, Keyword)
valid-types No [] Array of strings. Which value types the property accepts. See below. bool property_accepts_type(PropertyID, ValueType)

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 and logical-alias-for

These are two separate concepts, with unfortunately similar names:

  • 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 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. List all the properties that they can alias.

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-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.

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 CSSKeywordValue
  • 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

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>.

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

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