[css-properties-values-api] Add @Property. (w3c#847)

* [css-properties-values-api] Add @Property.

This adds basic support for @Property, without solving any of the
potentially complicated issues, like w3c#845 and w3c#846.

Resolves w3c#137, at least partially.

* Remove merge conflict things.

* Rewrapping, tiny wording tweaks, IDL/linking fixes

Author: andruud

css-properties-values-api/Overview.bs

@@ -850,37 +850,228 @@ CSSOM {#cssom}
 			return the result.
 </div>
 
+The <dfn>@property</dfn> rule {#at-property-rule}
+=================================================
+
+The ''@property'' rule provides an alternative way to register a custom property,
+directly in a stylesheet
+without having to run any JS.
+Valid ''@property'' rules result in a [=register a custom property|registered custom property=],
+as if {{registerProperty()}} had been called with equivalent parameters.
+
+The semantics of registered properties are the same
+regardless of the mechanism used to perform the registration.
+This means that, once registered,
+it does not matter whether the registration originated from {{registerProperty()}} or ''@property'':
+the property has the same behavior either way.
+
+The syntax of ''@property'' is:
+
+	<pre class="prod def" nohighlight>
+	@property <<custom-property-name>> {
+		<<declaration-list>>
+	}
+	</pre>
+
+''@property'' rules require a 'syntax' and 'inherits' descriptor;
+if either are missing,
+the entire rule is invalid and must be ignored.
+The 'initial-value' descriptor is optional
+only if the syntax is the [=universal syntax descriptor=],
+otherwise the descriptor is required;
+if it's missing, the entire rule is invalid and must be ignored.
+
+If an ''@property'' rule contains any unknown descriptors,
+the entire rule is invalid and must be ignored.
+
+Issue(845): What happens when we have both an ''@property'' rule, and a
+{{registerProperty()}} call for the same property?
+
+Issue(846): What happens when ''@property'' appears inside a shadow tree?
+
+
+The 'syntax' descriptor {#the-syntax-descriptor}
+------------------------------------------------
+
+	<pre class='descdef'>
+	Name: syntax
+	Value: <<string>>
+	For: @property
+	Initial: n/a (see prose)
+	</pre>
+
+Specifies the syntax of the custom property,
+in the form defined by [[#syntax-strings]].
+This descriptor is equivalent to the {{PropertyDescriptor/syntax|syntax}} member of {{PropertyDescriptor}}.
+
+The 'syntax' descriptor is required for the ''@property'' rule to be valid;
+if it's missing, the ''@property'' rule is invalid.
+
+If the provided string does not successfully [=consume a syntax definition|parse as a syntax definition=],
+the descriptor is invalid and must be ignored.
+
+
+The 'inherits' descriptor {#inherits-descriptor}
+------------------------------------------------
+
+	<pre class='descdef'>
+	Name: inherits
+	Value: true | false
+	For: @property
+	Initial: n/a (see prose)
+	</pre>
+
+Specifies whether or not the custom property inherits.
+This is equivalent to the {{PropertyDescriptor/inherits}} member of {{PropertyDescriptor}}.
+
+The inherits descriptor is required for the ''@property'' rule to be valid;
+if it's missing, the ''@property'' rule is invalid.
+
+
+The 'initial-value' descriptor {#initial-value-descriptor}
+----------------------------------------------------------
+
+	<pre class='descdef'>
+	Name: initial-value
+	Value: <<declaration-value>>
+	For: @property
+	Initial: the [=guaranteed-invalid value=] (but see prose)
+	</pre>
+
+Specifies the [=initial value=] of the custom property.
+This is equivalent to the {{PropertyDescriptor/initialValue}} member of {{PropertyDescriptor}}.
+
+If the value of the 'syntax' descriptor is the [=universal syntax descriptor=],
+then the 'initial-value' descriptor is optional.
+If omitted, the initial value of the property is the [=guaranteed-invalid value=].
+
+Otherwise,
+if the value of the 'syntax' descriptor is not the [=universal syntax descriptor=],
+the following conditions must be met for the the ''@property'' rule to be valid:
+
+	* The 'initial-value' descriptor must be present.
+	* The 'initial-value' descriptor's value must [=consume a syntax definition|parse successfully=]
+		according to the grammar specified by the [=syntax definition=].
+	* The 'initial-value' must be [=computationally independent=].
+
+If the above conditions are not met, the ''@property'' rule is invalid.
+
+
+Extensions to the {{CSSRule}} interface {#extensions-to-css-rule-interface}
+------------------------------------------------------------------------------------
+
+The {{CSSRule}} interface is extended as follows:
+
+<pre class='idl'>
+partial interface CSSRule {
+    const unsigned short PROPERTY_RULE = 18;
+};
+</pre>
+
+The <dfn interface>CSSPropertyRule</dfn> interface {#the-css-property-rule-interface}
+-----------------------------------------------------------------------------
+
+The {{CSSPropertyRule}} interface represents an ''@property'' rule.
+
+<pre class='idl' export>
+[Exposed=Window]
+interface CSSPropertyRule : CSSRule {
+    readonly attribute CSSOMString name;
+    readonly attribute CSSOMString syntax;
+    readonly attribute boolean inherits;
+    readonly attribute CSSOMString? initialValue;
+};
+</pre>
+
+<dl dfn-for=CSSPropertyRule dfn-type=attribute>
+	<dt><dfn>name</dfn>
+	<dd>
+		The custom property name associated with the ''@property'' rule.
+
+	<dt><dfn>syntax</dfn>
+	<dd>
+		The syntax associated with the ''@property'', exactly as specified.
+
+	<dt><dfn>inherits</dfn>
+	<dd>
+		The inherits descriptor associated withthe ''@property'' rule.
+
+	<dt><dfn>initialValue</dfn>
+	<dd>
+		The initial value associated with the ''@property'' rule,
+		which may not be present.
+</dl>
+
 
 Examples {#examples}
 ====================
 
 Example 1: Using custom properties to add animation behavior {#example-1}
 -------------------------------------------------------------------------
 
-<pre class='lang-markup'>
-&lt;script>
-CSS.registerProperty({
-	name: "--stop-color",
-	syntax: "&lt;color>",
-	inherits: false,
-	initialValue: "rgba(0,0,0,0)"
-});
-&lt;/script>
-
-&lt;style>
-
-.button {
-	--stop-color: red;
-	background: linear-gradient(var(--stop-color), black);
-	transition: --stop-color 1s;
-}
-
-.button:hover {
-	--stop-color: green;
-}
+<xmp class='lang-markup'>
+	<script>
+	CSS.registerProperty({
+		name: "--stop-color",
+		syntax: "<color>",
+		inherits: false,
+		initialValue: "rgba(0,0,0,0)"
+	});
+	</script>
 
-&lt;/style>
+	<style>
+	.button {
+		--stop-color: red;
+		background: linear-gradient(var(--stop-color), black);
+		transition: --stop-color 1s;
+	}
 
+	.button:hover {
+		--stop-color: green;
+	}
+	</style>
+</xmp>
+
+Example 2: Using ''@property'' to register a property {#example-2}
+------------------------------------------------------------------
+<xmp class='lang-markup'>
+	<script>
+		CSS.paintWorklet.addModule('circle.js');
+	</script>
+	<style>
+		@property --radius {
+			syntax: "<length>";
+			inherits: false;
+			initial-value: 0px;
+		}
+
+		div {
+			width: 100px;
+			height: 100px;
+			--radius: 10px;
+			background: paint(circle);
+			transition: --radius 1s;
+		}
+
+		div:hover {
+			--radius: 50px;
+		}
+	</style>
+	<div></div>
+</xmp>
+
+<pre class='lang-javascript'>
+	// circle.js
+	registerPaint('circle', class {
+			static get inputProperties() { return ['--radius']; }
+			paint(ctx, geom, properties) {
+				let radius = properties.get('--radius').value;
+				ctx.fillStyle = 'black';
+				ctx.beginPath();
+				ctx.arc(geom.width / 2, geom.height / 2, radius, 0, 2 * Math.PI);
+				ctx.fill();
+			}
+	});
 </pre>
 
 Security Considerations {#security-considerations}