Let's you have a page with a relatively strict doctype and HTML markup that's pretty close to compliant, but perhaps misses in a few silly ways that are out of your control.
How can you tell (or: what will determine) when the browser decides to go into "quirks" mode rather than use it's more standards compliant engine?
I'm looking for answers for each of the major browsers, since IE, Chrome, Safari, and Firefox will of course all handle that differently. Is one single error enough to force it or do you have some leeway?