There's also the one-line-at-a-time approach. Strip the page down to nothing but the dtd, html and body tags. Then add things one by one and see when it breaks.
About half of my own pages cause MSIE 5.23 to crash. Not because they are hifalutin overly fancy non-responsive pages, no sir, it's because they use TWO external stylesheets. That's one more than the browser can handle. Pity, because an antique browser is a pretty handy way to test for overall responsiveness and degrades-gracefully-ness.
Now, if they made a browser that refused to open pages with more than three external style sheets... For some reason I keep meeting sites with exactly seventeen. D'you think they pick this number on purpose?