vim-patch:43725ac: runtime(css): Add additional CSS pseudo-classes and functions (#42192)

Problem:  Other than `state()` these are widely supported CSS features
Solution: Add :host, :host-context(), slotted(), and state() CSS
          pseudo-classes and functions (S0AndS0).

Reference:
- https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/:host
- https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/:host_function
- https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/::slotted
- https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/:state

TLDR: `host` and `host-context()` are useful for shadowDom stuff,
`::slotted()` enables targeting elements passed/injected into a
`<template>` and `state()` enables targeting custom identifiers.

For example, currently I'm (ab)using `:host` as a drop-in replacement
for `:root` when defining variables that are intended to be shared
across custom/template elements;

- `./assets/css/host.css`
   ```css
   :host {
     --default__margin-block: 0.25rem;
   }
   ```
- `./custom-elements/example.html`
   ```html
   <template>
     <link rel="stylesheet" href="/assets/css/host.css">

     <style>
       header {
         margin-block-start: calc(var(--default__margin-block) * 2);
       }
     </style>

     <header></header>
   </template>
   ```

closes: vim/vim#21404

https://github.com/vim/vim/commit/43725acce680d1b999e22fad68cea99c2b593325

Co-authored-by: S0AndS0 <strangerthanbland@gmail.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/runtime/syntax/css.vim b/runtime/syntax/css.vim
index b3510229e7..9c7e306413 100644
--- a/runtime/syntax/css.vim
+++ b/runtime/syntax/css.vim
@@ -12,6 +12,7 @@
 " 2025 Nov 11: improve support for cssBoxProperties #18717
 " 2026 Feb 13: add missing numeric units (baseline 2015, 2020, 2022, 2023, 2026) #19325
 " 2026 Jun 28: add scrollbar-gutter properties
+" 2026 Sep 30: add :host, :host-context(), slotted(), and state() #21404
 
 " quit when a syntax file was already loaded
 if !exists("main_syntax")
@@ -479,12 +480,12 @@ syn match cssAttrComma ","
 " Pseudo class
 " https://www.w3.org/TR/selectors-4/
 syn match cssPseudoClass ":[A-Za-z0-9_-]*" contains=cssNoise,cssPseudoClassId,cssUnicodeEscape,cssVendor,cssPseudoClassFn
-syn keyword cssPseudoClassId contained link visited active hover before after left right
-syn keyword cssPseudoClassId contained root empty target enabled disabled checked invalid
+syn keyword cssPseudoClassId contained link visited active hover before after left right slotted
+syn keyword cssPseudoClassId contained root host empty target enabled disabled checked invalid
 syn match cssPseudoClassId contained "\<first-\%(line\|letter\)\>"
 syn match cssPseudoClassId contained "\<\%(first\|last\|only\)-\%(of-type\|child\)\>"
 syn match cssPseudoClassId contained  "\<focus\%(-within\|-visible\)\=\>"
-syn region cssPseudoClassFn contained matchgroup=cssFunctionName start="\<\%(not\|is\|lang\|\%(nth\|nth-last\)-\%(of-type\|child\)\)(" end=")" contains=cssStringQ,cssStringQQ,cssTagName,cssAttributeSelector,cssClassName,cssIdentifier
+syn region cssPseudoClassFn contained matchgroup=cssFunctionName start="\<\%(not\|is\|lang\|host\|host-context\|state\|\%(nth\|nth-last\)-\%(of-type\|child\)\)(" end=")" contains=cssStringQ,cssStringQQ,cssTagName,cssAttributeSelector,cssClassName,cssIdentifier
 " ------------------------------------
 " Vendor specific properties
 syn match cssPseudoClassId contained  "\<selection\>"