Inb4 youtube is bad for a million other reasons. Yes I know, most of them are ameliorated by extremely heavy adblocking, that's not what this is about.
In the last couple days, youtube has completely forgotten how to format a basic grid layout. Look at those inconsistent gaps between thumbnails, look at those misaligned columns, what the fuck is going on here?
What I suspect is happening here is that it's using a flexbox instead of a grid. I'm guessing it has a flex direction of row (horizontal left to right), uses flex wrap, and has each video thumbnail have a flex basis to keep them around the same size. It probably makes it easier to look good enough at most screen sizes without needing to have a bunch of media queries change the number of columns at different screen sizes, at the cost of things not always being perfectly aligned. Probably a good tradeoff compared to the alternative tbh.