Comment by skydhash

> You also need the converse - find which elements a given style applies to - and there's no way to do that AFAIK.

I've never needed to do this, because I pay attention to my DOM structure and from CSS selectors can figure where a style applies. But I've just checked and the search bar for Firefox Inspector supports css selectors.

> In my experience the DOM doesn't have semantics, or to the extent that it does, they change all the time.

The DOM semantics are those of a hyper linked documents|forms. Take a page and think about what each elements means and their relations to each order. They will form a hierarchy with some generic components replicated. The due to how CSS is applied, you go from generic to specific elements, using the semantic structure to do the targeting

As an example, the structure of HN's reply page is

  page
    header
      logo + Title
    body
      comment_box
        upvote_button + comment_metadata
        comment_text
      textbox // reply text
      button // reply

This and the structure of the other pages will give you an insight on how to target the relevant elements.