mattwoodrow
commented
6 months ago It looks like the current behaviour in blink/gecko is that the canvas surface color is included in the captured backdrop if no other 'backdrop root' properties are present (like opacity), but is not included if they are.
What is the expected behaviour if the backdrop-filter property is set on the document element itself?
It looks like many of the current WPT tests are capturing the backdrop of a document without a background-color value set anywhere, and expecting to capture white (see backdrop-filter-basic.html)
My understanding is that the default background color is transparent, and it's only the UA-specific 'canvas surface' that would be providing a white background color in this case - https://www.w3.org/TR/css-backgrounds-3/#special-backgrounds.
We also likely want to clarify what happens when there is a background on the root element (or \<body>), but it gets propagated to the canvas background. If the root element itself is the backdrop root, then I'd expect the propagated background to not be included in the captured backdrop.