[css-position] Insets don't reduce the cb when sizing an abspos

[Loirooriol]

From https://drafts.csswg.org/css-position/#abspos-insets

For an absolutely positioned box, the inset properties effectively reduce the containing block into which it is sized and positioned by the specified amounts.

And https://drafts.csswg.org/css-position/#abspos-layout goes on with

1. First, its inset-modified containing block is calculated, defining its available space. (See § 3.5 Absolute (and Fixed) Positioning.) [...]
2. Next, its width and height are resolved against this definite available space [...]

But that's not actually the case. Percentages are resolved with respect to the original containing block size, not the inset-modified one. E.g.

#cb {
  position: relative;
  width: 100px;
}
#abspos {
  position: absolute;
  left: 25px;
  right: 25px;
  width: 100%;
  height: 100px;
  background: blue;
}

<div id="cb">
  <div id="abspos"></div>
</div>

results in a 100x100 square, not a 50x100 rectangle. https://software.hixie.ch/utilities/js/live-dom-viewer/saved/8114

[Loirooriol]

I guess the "width and height are resolved against this definite available space" refer to an auto width/height. But as currently stated it's misleading.

[fantasai]

@Loirooriol Thanks for catching this. I added a sentence to except percentages. (Didn't want to make the general rule specific to auto because it will also apply to the fit-content and stretch keywords.) Let me know if the edits look good.

[Loirooriol]

Seems fine, but I have tested fit-content and stretch, and we don't have interoperability:

• In Firefox, -moz-fit-content and -moz-available use the inset-modified area indeed.
• In Chromium and WebKit, fit-content and -webkit-fill-available use the original area, like percentages.

https://software.hixie.ch/utilities/js/live-dom-viewer/saved/9601