[whatwg] [CSSWG][selectors-4] Updated WD of Selectors L4

[fantasai]

The CSS WG has published an updated Working Draft of Selectors Level 4:

   https://www.w3.org/TR/selectors-4/

Selectors are patterns that match against elements in a tree and are used
as a core part of CSS and in DOM methods such as .querySelector()

This update adds, drops, and renames a number of selectors:
   * zero-specificity pseudo-class named :where()
   * :matches() renamed to :is()
   * :blank defined to select empty user input elements
   * :empty redefined to ignore whitespace-only text nodes
   * :drop() dropped due to removal of support in HTML
   * Added case-sensitive attribute value matching flag

In addition, the specificity rules for :is() and :nth-child() were altered
to use the most specific selector argument rather than the most specific
selector that happened to match. See
   https://www.w3.org/TR/selectors-4/#specificity-rules
and discussion in
   https://github.com/w3c/csswg-drafts/issues/1027

Changes since the February 2018 WD are all listed at:
    https://www.w3.org/TR/2018/WD-selectors-4-20181121/#changes

One major issue that's open is redefining the way invalid selectors are
handled within `:is()` and similar pseudo-classes to ignore unknown
selectors rather than invalidating the entire style rule. See
   https://github.com/w3c/csswg-drafts/issues/3264

Another series of open issues concerns the :visited pseudo-class and
how to balance security concerns with usability requirements. See e.g.
   https://github.com/w3c/csswg-drafts/issues/3012
   https://github.com/w3c/csswg-drafts/issues/2263

Please review the draft, and send any comments to the CSSWG mailing list,
<www-style at w3.org>, prefixed with [selectors-4] (as I did on this
message) or (preferably) file them in the GitHub repository at
   https://github.com/w3c/csswg-drafts/issues

For the CSS WG,
~fantasai