[css-display] Define interaction of display:contents and replaced elements

[zcorpan]

https://drafts.csswg.org/css-display/#box-generation

w3c/csswg-test#1129

It was not clear to me from the spec what the expected result of applying display:contents to replaced elements is. The added test for replace contents matches what Gecko implemented.

The implemented behavior seems to be that display:contents on replaced elements and form controls (not all of those are actually replaced elements) is ignored, as far as I can tell.

cc @rune-opera @tabatkins @fantasai

[MatsPalmgren]

We ignore display:contents in a few cases where it doesn't make sense, such as
for replaced elements. (I can dig up the exact conditions if you need it.)

So I think this change is basically wrong:
c65b958
rather than referring to literal display values, it would be better to use more general terms,
e.g. "when the element has a principal box, use its size, otherwise use the computed value"

[fantasai]

The CSSWG decided to treat display: contents as display: inline on replaced elements and form controls. Edited in c588376 by making it compute to inline on such elements. Please let me know if you'd rather do something more magical.

[zcorpan]

LGTM

[MatsPalmgren]

I fully agree with bz in #1024 (comment)

The spec should say that contents computes to contents but the used value is inline for these elements.

In general, adding new spec language that makes a computed value depend on layout stuff such as box construction is pretty much always bad.