TYPO3 Core

Steps to reproduce:

1. Create and apply an RTE configuration with contentsCss.
2. In the referenced CSS file, declare CSS using :has and or :where selectors.
3. Edit content in the backend, and inspect with the browser devtools. Selectors with :has and :where will not be applied. They seem to be missing in the constructed CSS.

Example:

ul li:has(& > p.text-center) {
  color: red;
}

Then, in the editor, create a list and center it. The list item will not be colored red. The devtools web inspector will not show any sign of these selectors.

Note that this isn't a nesting standard incompatibility. This does not work either:

ul li:has(> p.text-center) {
  color: red;
}

Note that neither :has() nor :where() selectors work, but pseudo-classes like ::before do.

According to AST explorer, css-tree, the package used to preprocess editor CSS content, parses the :has selector just fine: https://astexplorer.net/#/gist/244e2fb4da940df52bf0f4b94277db44/8f51276c0fcd8ce562da7cc695fb33d0b2fbef1a