CSS - @property

Quiz

The @property CSS at-rule, part of the CSS Houdini APIs, allows developers to explicitly define custom properties.

The @property rule:

Provides features such as property type checking, constraints, setting default values, and specifying inheritance behavior for CSS custom properties.
Allows you to register custom properties in stylesheets without using JavaScript.
Create registered custom properties, similar to calling registerProperty() with equivalent parameters in JavaScript.

Some points to remeber when using @property CSS at-rule:

A valid @property rule means registering a custom property with the property name derived from the serialization in the rule's prelude.
@property rules must contain both a syntax and inherits descriptor. If either is missing, the rule is invalid and ignored.
The initial value descriptor is optional only for universal syntax definition, otherwise, it is required. If it is missing, the entire rule is invalid and ignored.
Unknown descriptors within a @property rule are ignored and considered invalid, but they do not invalidate the entire @property rule.

Syntax

@property <custom-property-name> { <declaration-list> }

CSS @property - Using Custom Property

The following example demonstrates the @property without using javascript.

<html>
<head>
<style>
   @property --main-color {
      syntax: '<color>';
      inital-value: black;
      inherits: true;
   }
   .box {
      --main-color: red;
      background-color: var(--main-color);
      width: 200px;
      height: 200px;
      margin: 50px auto;
      display: flex;
      align-items: center;
      justify-content: center;
      color: white;
      font-size: 24px;
      font-weight: bold;
      text-align: center;
   }
</style>
</head>
<body>
   <div class="box">
      <p>CSS At-Rules @property</p>
   </div>
</body>
</html>

CSS @property - Using Custom Property And registerProperty()

The following example demonstrates the usage of @property and registerProperty().

In the following example, CSS @property defines a custom property called --item-size and specifies that it accepts and inherits percentage values and starts with an initial value of 40%.
The window.CSS.registerProperty in JavaScript dynamically registers a new custom property named --item-color, sets its syntax to <color>, specifies that it is not inherited, and assigns it an initial value of peachpuff.

<html>
<head>
<style>
   @property --item-size {
      syntax: "<percentage>";
      inherits: true;
      initial-value: 40%;
   }
   /* set custom property values on parent */
   .container {
      display: flex;
      height: 100px;
      border: 3px dotted black;
      --item-size: 50%;
      --item-color: tomato;
      --item-border: 5px solid green;
   }
   /* use custom properties to set item size and background color */
   .item {
      width: var(--item-size);
      height: var(--item-size);
      border: var(--item-border);
      background-color: var(--item-color);
   }
   /* set custom property values on element itself */
   .second {
      --item-size: initial;
      --item-color: inherit;
   }
</style>
</head>
<body>
   <div class="container">
      <div class="item">First Item</div>
      <div class="item">Second Item</div>      
   </div>
<script>
   window.CSS.registerProperty({
   name: "--item-color",
   syntax: "<color>",
   inherits: false,
   initialValue: "peachpuff",
   });
</script>
</body>
</html>

Descriptors

Descriptors	Explaination
syntax	Explains the permissible syntax for the property.
inherit	Determines if the custom property registration defined by @property inherits by default.
initial-value	Defines the property's initial value.

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