[css-color-adjust-1] Force opaque background if color-adjust mismatches between iframe and embedded doc. #4472

Author: fantasai

css-color-adjust-1/Overview.bs

@@ -70,7 +70,7 @@ Preferred Color Schemes {#preferred}
 
 	* the initial color of the background canvas <span class=issue>find what we name this in other specs</span>,
 		the initial value of the '/color' property,
-		and the [[css-color-4#system-colors|system colors]]
+		and the [[css-color-4#css-system-colors|system colors]]
 	* the default colors of scrollbars and other interaction UI
 	* the default colors of form controls and other "specially-rendered" elements
 	* the default colors of other browser-provided UI, such as "spellcheck" underlines
@@ -215,16 +215,32 @@ Finding the Used Color Scheme {#color-scheme-processing}
 	written before color scheme preferences were exposed,
 	do not change behavior.
 
+Effects of the Used Color Scheme {#color-scheme-effect}
+--------------------------------
+
 	For all elements,
-	rendering with a color scheme should affect the colors used in all browser-provided UI for the element--
+	rendering with a color scheme should cause the colors used in all browser-provided UI for the element--
 	e.g. scrollbars, spellcheck underlines, form controls, etc.--
 	to match with the intent of the color scheme.
 
-	For the root element of a page,
+	On the root element,
 	rendering with a color scheme additionally must affect the background of the canvas,
 	the initial value of the '/color' property,
-	and the [[css-color-4#system-colors|system colors]],
-	and should affect the page's scrollbars.
+	and the used values of the [[css-color-4#css-system-colors|system colors]],
+	and should also affect the viewport's scrollbars.
+
+	In order to preserve expected color contrasts,
+	in the case of embedded documents typically rendered over a transparent [=canvas=]
+	(such as provided via an HTML <{iframe}> element),
+	if the used 'color-scheme' of the element
+	and the used 'color-scheme' of the embedded document’s root element
+	do not match,
+	then the UA must use an opaque [=canvas=] of the ''Canvas'' color
+	appropriate to the embedded document’s 'color-scheme'
+	instead of a transparent canvas.
+	This rule does not apply to documents embedded
+	via elements intended for graphics
+	(such as <{img}> elements embedding an SVG document).
 
 The "color-scheme" <{meta}> value {#color-scheme-meta}
 ---------------------------------