Agenda+ CSS Pseudo-Elements L4 Issues, Review

So I worked on CSS Pseudo-elements for a bit, and the following things
need WG review and input:

1. Switching ::selection to use inheritance rather than cascade
    for parent-child value propagation

    We resolved on this in
      https://github.com/w3c/csswg-drafts/issues/2474#issuecomment-380369965
    Edits can be seen at
      https://drafts.csswg.org/css-pseudo-4/#highlight-cascade

    This is a request for review from the WG, prior to requesting republication.
    Follow-up issues should be filed separately, unless you think the edits
    fundamentally don't reflect the resolution or you want to re-open the
    discussion.

2. Color of text / decorations

    It seemed expedient for the previous issue to define 'currentColor'
    as a keyword representing the color the text would have if ::selection
    were unstyled. So https://github.com/w3c/csswg-drafts/issues/2850 asks
    confirmation for two things:
      a) That the behavior clarified in that issue is what we want.
      b) Whether the CSSWG thinks defining currentColor in this way is OK.

    See spec at https://drafts.csswg.org/css-pseudo-4/#highlight-painting

3. CSSPseudoElement issues
    https://drafts.csswg.org/css-pseudo-4/#CSSPseudoElement-interface

    https://github.com/w3c/csswg-drafts/issues/2816 to access originating Element
    https://github.com/w3c/csswg-drafts/issues/2815 syntax of type values

4. Applying stroke/fill to ::selection

    See https://github.com/w3c/csswg-drafts/issues/2362#issuecomment-450426449

5. Applying ::placeholder / :placeholder-shown on placeholders that are
    elements (rather than attributes) in HTML.

    https://github.com/w3c/csswg-drafts/issues/2517

~fantasai