Editorial: replace "solid black" with "opaque black"

Fixes #3431.

Author: TimvdLippe

source

@@ -65505,7 +65505,7 @@ interface <dfn>OffscreenCanvasRenderingContext2D</dfn> {
   lowercase</span> before establishing if they support that type.</p>
 
   <p>For image types that do not support an alpha channel, the serialized image must be the bitmap
-  image composited onto a solid black background using the source-over operator.</p>
+  image composited onto an <span>opaque black</span> background using the source-over operator.</p>
 
   <p>If <var>type</var> is an image format that supports variable quality (such as
   "<code>image/jpeg</code>") and <var>quality</var> is given, then, if <span