I was under the impression that "DOM ready" did not wait for CSS, but that "load" did. In fact, i was also under the impression that CSS (loaded in the <head> at least) blocked any further processing until it's downloaded. Am I wrong?
I'm not sure about ready vs load, but that would make sense.
As for downloading CSS -- it won't prevent your JS from executing, but it'll block page rendering until all the stylesheets have been downloaded and parsed.
DOMContentLoaded, in all important browsers, does block to wait for CSS. Manipulating the DOM before this event both increases the chance to make changes before first-paint (decreases "jigger" on page load) and gives you the chance to request more resources earlier.
I guess I never really noticed this because my scripts are always included so much later in the document than my CSS.
It most surely will, in at least Gecko and WebKit. Too many pages have JS that depends on all preceding stylesheets having been loaded.