[css-nesting][css-style-attr] Allow CSS nesting for inline styles

Whether CSS nesting works for inline styles isn't actually defined in the specs, as far as I can find.

So I was under the assumption that I could do:

<ul style="
   & a {
    font-weight:bold;
    &:link {color:green}
    &:is(:hover, :active) {color:red} 
  }">
  <li><a href="#">Home</a></li>
  <li><a href="#">Blog</a></li>
  <li><a href="#">About</a></li>
 </ul>

And found out that no, it doesn't work.

I'm hoping that this behaviour can be defined at the very least, and allowed if at all possible.

By allowing nesting to work with inline styles, it creates a very useful method to scope CSS styles.

Start with issue #8752 and its 23-comment discussion about CSS nesting in the style attribute. Review the CSS nesting and inline-style specifications referenced or discussed there; done means reaching a clear specification decision that defines whether nested inline styles are supported and how they behave.