[mediaqueries-5] Per WG resolution, embedded SVGs use the used color …

…scheme of the embedding node for prefers-color-scheme. #7213

mediaqueries-5/Overview.bs

@@ -3041,6 +3041,34 @@ Detecting the desire for light or dark color schemes: the 'prefers-color-scheme'
 	so that 'prefers-color-scheme' reflects preferences appropriate to the medium
 	rather than preferences taken out of context.
 
+	If evaluated in an embedded SVG document
+	using the "Secure Animated" embedding mode,
+	the preferred color scheme must reflect the value of the [=used color scheme=]
+	on the embedding node in the embedding document.
+
+	<details class=note>
+		<summary>Why do this?</summary>
+
+		While the outermost document needs to get the user's preference directly,
+		it's more useful for an embedded document
+		to use the color scheme of its surrounding embedding context,
+		so it matches the surrounding content.
+
+		However, this enables communication
+		from the embedding document to the embedded document,
+		so it's currently restricted to SVG's
+		using the "Secure Animated" mode,
+		which can't load external resources
+		or run script,
+		and thus can't respond to the color scheme
+		in any way observable to the outside world.
+
+		Whether or not to do similar for iframes,
+		and under what conditions,
+		is being discussed in
+		<a href="https://github.com/w3c/csswg-drafts/issues/7213/">Issue 7213</a>.
+	</details>
+
 	<div class=note>
 		This feature, like the other 'prefers-*' features,
 		previously had a <css>no-preference</css> value