Is DOM rendering GUARANTEED to block during a single (synchronous) function’s execution?

DOM blocking is something many people not familiar with JavaScript’s strictly single-threaded synchronous execution model find out about the hard way, and it’s usually just something we want to work around somehow (using timeouts, web-workers, etc). All well and good. However, I would like to know if blocking of the actual user-visible rendering is something you can actually rely on. I’m 90% sure it is de facto the case in most browsers but I am hoping this isn’t just a happily consistent accident. I can’t seem to find any definitive statements from DOM specifications or even vendor documentation like MDM.