<style> should use child text content changed steps #2996

Currently it says

The element is not on the stack of open elements of an HTML parser or XML parser, and one of its child nodes is modified by a script.

This should be formalized, probably by using the somewhat-new child text content changed steps. However, this would probably depend on #2995, since XML-based styling languages need something more like "descendant changed steps".

See also:

For styling languages that consist of pure text (as opposed to XML), a style element's style data is the child text content of the style element. For XML-based styling languages, the style data consists of all the child nodes of the style element.

It's also worth testing that it truly is child text content, and not descendant text content (i.e. the value of textContent), by using DOM manipulation APIs to insert child nodes that wrap extra style rules (e.g. <span>s or even <style>s), and ensuring that those rules are not applied.

If we can resolve #2995 one way or another we can morph this into a "good first bug".