[css-typed-om] computedStyleMap()

[annevk]

It'd be great to be a bit more restrictive here as to when it returns values. Does this work for

• display:none
• Elements outside the tree
• Elements in documents that are not rendered

and do we have tests covering all these scenarios? (It's worth going through the open getComputedStyle() issues to get some inspiration.)

[annevk]

(Some overlap with #659 here.)

[darrnshn]

There are currently no tests for these cases unfortunately, pending resolution of #659.