If you add some kind of "snap to DOM" ability or whatever, so that the rulers align with "places" already "defined" by the DOM+CSS, it might even be better.
i actually have something like this already made (with guides and all) that i've been dragging my feet (no pun intended) to open source since it's a bit rough around the edges. i'll put it up in the next week or so. feel free to watch the repo in the likely case it'll not make it to the front page.
But it may not, obviosly.