[csswg-drafts] [css-contain] contain:paint should use padding box instead of content box

trchen1033 has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-contain] contain:paint should use padding box instead of content box ==
@tabatkins

In section 3.4. Paint Containment: "The contents of the element must be clipped to the element’s content box,"
https://www.w3.org/TR/css-contain-1/#containment-paint
https://drafts.csswg.org/css-contain/#containment-paint

Instead of the content box, the actual implementation in Blink uses the padding box, which is more inline with overflow clip.
Also Blink currently doesn't apply inner border radius clip to paint containment, which I feel weird. We should decide what is the intended behavior and document that in the spec, then implement it.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/2223 using your GitHub account