I think the best way to represent comment threads with semantic HTML would be using the <ul> element. So the top-level comment thread for a post goes in a <ul>

I think the best way to represent comment threads with semantic HTML would be using the <ul> element. So the top-level comment thread for a post goes in a <ul> element, and then there's a nested <ul> element for each subthread. Of course, you can use CSS to drop the bullets and adjust the amount of indentation. Then, a screen reader or browser extension can easily provide a way to skip over whole subthreads, without having to do something HN-specific.