Intent to implement and ship: CSS ::marker pseudo-element

[Mats Palmgren]

Summary:
The ::marker pseudo-element represents the automatically generated
marker box of a list item. It allows authors to style the marker
and generate content for it through its 'content' property (like
::before/::after).

Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=205202
Animation support is pending:
https://bugzilla.mozilla.org/show_bug.cgi?id=1538618

Link to standard:
https://drafts.csswg.org/css-pseudo-4/#marker-pseudo
https://drafts.csswg.org/css-lists/#markers
https://github.com/w3c/csswg-drafts/issues/3499

Platform coverage: All platforms

Estimated or target release: Firefox 68

Preference behind which this will be implemented: none

DevTools bug: none (I've checked that a ::marker shows up as
a pseudo in the panel with ::before/::after and it appears
to work fine)

Do other browser engines implement this?
Safari implements limited support for ::marker (it doesn't support
the 'content' property, and it has no support for the built-in
'list-item' counter, both which are major pieces of this general
feature). Chrome supports none of it AFAICT.
https://wpt.fyi/results/css/css-pseudo

web-platform-tests:
There are some tests in css/css-pseudo/ and I've added
a bunch more.

Is this feature restricted to secure contexts? No

Many thanks to Emilio who patiently reviewed all of these list
item related changes.


/Mats

[Mats Palmgren]