[css-values] Per f2f resolution, add special 'guaranteed to be a local ref' handling of fragment-only URLs.

Author: tabatkins

css-values/Overview.bs

@@ -22,7 +22,7 @@ Editor: fantasai, http://fantasai.inkedblade.net/contact
 Former Editor: Håkon Wium Lie, Opera Software, howcome@opera.com
 Abstract: This CSS3 module describes the common values and units that CSS properties accept and the syntax used for describing them in CSS property definitions.
 At Risk: ''toggle()'', ''attr()''
-Ignored Terms: <spacing-limit>, containing block
+Ignored Terms: <spacing-limit>, containing block, linear-gradient
 Ignored Vars: Cn+1, n
 </pre>
 <pre class='link-defaults'>
@@ -574,6 +574,54 @@ Relative URLs</h4>
 		The same image will be used regardless of the URL of the source document containing the <code>&lt;body></code>.
 	</div>
 
+<h4 id='local-urls'>
+Fragment URLs</h4>
+
+	In some CSS-using languages like SVG,
+	one often needs to refer to elements in the host document directly
+	(such as to reference a <{linear-gradient}> element).
+	To facilitate this and work around some common eccentriticites in browser URL handling,
+	CSS has special behavior for fragment-only urls.
+
+	If a ''url()''’s value starts with a U+0023 NUMBER SIGN (<code>#</code>) character,
+	parse it as per normal for URLs,
+	but additionally set the <dfn export for="url()">local url flag</dfn> of the ''url()''.
+
+	When matching a ''url()'' with the <a>local url flag</a> set,
+	ignore everything but the URL's fragment,
+	and resolve that fragment against the host document.
+	This reference must always be treated as same-document
+	(rather than cross-document).
+
+	Note: This means that an SVG reference like ''url(#my-gradient)''
+	will work the same whether the CSS is written inline in the SVG document
+	or linked in as an external CSS stylesheet.
+
+	When serializing a ''url()'' with the <a>local url flag</a> set,
+	it must serialize as just the fragment.
+
+	<details class=note>
+		<summary>What "browser eccentricities"?</summary>
+
+		Theoretically, browsers should re-resolve any relative URLs,
+		including fragment-only URLs,
+		whenever the document's base URL changes
+		(such as through mutation of the <{base}> element,
+		or calling {{History/pushState()}}).
+		In many cases they don't, however,
+		and so without special handling,
+		fragment-only URLs will suddenly become cross-document references
+		(pointing at the previous base URL)
+		and break in many of the places they're used.
+
+		Refusing to re-resolve relative URLs makes sense in many cases
+		(it will usually give the <em>desired</em> behavior
+		when the URL is path-relative),
+		but fragment-only URLs express a clear semantic
+		of wanting to refer to the current document
+		regardless of what its URL is.
+	</details>
+
 <h4 id='url-modifiers'>
 URL Modifiers</h4>