[cssom] getComputedStyle for non-existent pseudo-element

[zcorpan]

https://drafts.csswg.org/cssom/#resolved-values

width
...
If the property applies to the element or pseudo-element and the resolved value of the display property is not none or contents, then the resolved value is the used value. Otherwise the resolved value is the computed value.

See web-platform-tests/wpt#7020

What should happen for getComputedStyle(el, '::before').width if el has no ::before box generated? This appears to not be particularly well defined.

https://drafts.csswg.org/cssom/#terminology says

In this specification the ::before and ::after pseudo-elements are assumed to exist for all elements even if no box is generated for them.

[Loirooriol]

I would expect the same behavior as when an element does not generate boxes due to display: none or display: contents. In fact the spec is also missing the case of an element whose display is not none or contents but no box is generated due to a display: none ancestor. Then, all Firefox, Chrome and Edge return the computed value, but according to the spec it seems it should be the used value.

[tabatkins]

Yup, it should be the same as a display:none element. (In other words, the pseudo-element always exists, but most of the time just doesn't generate a box due to content.)

[FremyCompany]

Could probably be merged with #2149