Make UA stylesheet bits from css-overflow spec explicit.

[emilio]

What is the issue with the HTML Standard?

The CSS Overflow 4 spec has a hand-wavy section about UA rules in https://drafts.csswg.org/css-overflow-4/#overflow-control:

Host languages should define UA style sheet rules that apply a default value of clip to such elements and set their overflow-clip-margin to content-box.

In practical terms, for blink, that means this stylesheet:

video, canvas, img {
  overflow: clip;
  overflow-clip-margin: content-box;
}

iframe, embed, object {
   overflow: clip !important;
   overflow-clip-margin: content-box !important;
}

It's not clear why some are !important and some are not. Also, it seems this is missing <input type=image> at the very least?

HTML should be explicit about this.

cc @khushalsagar @bfgeek @lilles @zcorpan @annevk @whatwg/css

[emilio]

While looking into implementing this in Firefox, I found that having that CSS rule for <object> breaks Acid2. And it only works by chance in Chromium: https://issues.chromium.org/issues/465987920