CSS Properties and Values API Level 1

This CSS module defines an API for registering new CSS properties. Properties registered using this API are provided with a parse syntax that defines a type, inheritance behaviour, and an initial value.

1. Introduction

CSS defines a comprehensive set of properties that can be manipulated in order to modify the layout, paint, or behaviour of a web document. However, web authors frequently wish to extend this set with additional properties.

[css-variables] provides primitive means for defining user-controlled properties, however these properties always take token lists as values, must always inherit, and can only impact document layout or paint by being re-incorporated into the value of other properties via a var() reference.

This specification extends [css-variables], allowing the registration of properties that have a value type, an initial value, and a defined inheritance behaviour.

This specification is complementary to [css-paint-api] and [css-layout-api], which allow custom properties to directly impact paint and layout behaviours respectively.

2. Registering custom properties

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