Arrange the html so that the list of comments is at the end (via css). Keep the http connection open, have the show more button send some of request, and when you receive that request send the rest of the page over the original http connection.

As usual, solve people problems via people, not tech.

Maybe css to load an image on :active or is there some better way?

① A submit button or link targeting an iframe which is visually hidden. (Or even don’t hide it. If only seamless iframes had happened, or any other way of auto-resizing an iframe: relevant spec issues are https://github.com/whatwg/html/issues/555 and https://github.com/w3c/csswg-drafts/issues/1771.)

② A submit button or link to a URL that returns status 204 No Content.

(CSS image loading in any form is not as robust because some clients will have images disabled. background-image is probably (unverified claim!) less robust than pseudoelement content as accessibility modes (like high contrast) are more likely to strip background images, though I’m not sure if they are skipped outright or load and aren’t shown. :active is neither robust nor correct: it doesn’t respond to keyboard activation, and it’s triggered on mouse down rather than mouse up. Little tip here for a thing that people often get wrong: mouse things activate on mouseup, keyboard things on keydown.)

.button:active { background-image: url('/some-reference-thats-actually-a-tracker'); }

“View entire discussion” couldn’t be implemented perfectly with <details> in its present form, but you can get quite close to it with a couple of different approaches.

I think the infinite scrolling of subreddits is about the only thing that would really be lost by shedding JavaScript. Even inline replies can be implemented quite successfully with <details> if you really want.