In developing posthtml-postcss-inline, I reached the point where it can't inline complex CSS selectors, because posthtml-match-helper doesn't support that. The problem is discussed here, and it was concluded that it's a feature that should be implemented in the tree.match() API method of PostHTML.

We need a way to match elements based on complex CSS selectors.

For example, imagine this with posthtml-match-helper:

matchHelper("div > span")