Define what happens to out-of-flow positioned elements.

[bfgeek]

See a simple test case:
https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=6699

Safari & Firefox differ.

Here there are a few sub-questions:

1. Can MathML elements becomes out-of-flow?
2. Can MathML elements be containing-blocks for out-of-flow elements?
3. What this the static-position for OOF elements?

(2) Has some complexities, e.g. if they aren't allowed to be containing-blocks for OOFs, you also need to define that things that implicitly define containing-blocks, e.g. filter() aren't allowed on MathML elements.

[bfgeek]

My personal preference (subject to change) would probably be to allow OOFs on any element, and allow for any element to be a containing-block.

But this depends on other decisions.

The static-position probably wants to follow flex/grid.

This needs tests with various elements, and various combinations of containing blocks.
In addition there needs to be tests with fixed, etc.

[fred-wang]

See also #16

[fred-wang]

Closing since it's similar to #16