Maybe it could be interesting to be able to visualize child rules from a parent rule

Let's take this example:

body {
 background: tomato;
 & h1 {
  color: gold;
 }
}

Selecting the body (after Bug 1838011 is done), we'll show

body {
  background: tomato;
}

But we could maybe display

body {
  background: tomato;
  & h1 { ▶︎ }
}

and clicking the ▶︎ would show the nested rule content
If we're concerned it might take too much space in case there are many nested rules, we could group them in a dedicated section

body {
  background: tomato;
  (nested rules ▶︎)
}

It maybe be nice to allow modifying properties / selectors of nested rules so you could edit your CSS pretty fast
Maybe this is a bit redundant with the style editor, but when in the rule view, you have more context (e.g. you can see the DOM tree)