Use Layout/Box tree order instead of DOM Tree order

[jamesnw]

Per @mfreed7- " What matters is that the anchor element gets laid out first, and the anchored/positioned element gets laid out after that."

DOM and Layout/Box tree order are quite similar, but aren't identical. For a counter example, see https://github.com/web-platform-tests/wpt/blob/master/css/css-anchor-position/anchor-position-top-layer-005.html.

#207

Related:

• Support absolutely positioned anchors (not just top layer) #103 (comment)

• [css-anchor-position] Absolutely positioned element does not find fixed anchor w3c/csswg-drafts#10419

• Update validity test assumptions #207

• Containing Block Spec

• Unskip test from 5ab4ea4