That's pretty good too

Shorter lines are easier to read because it's easier to find the beginning of the next one. Rule of thumb is indeed a maximum of about 80 characters, go take a random printed book and see how long the lines are they're like that for a reason. (Newspapers are shorter because smaller print, also, more opportunities for headlinest).

The contrast and line spacing stuff -- debatable. But adjusting line-width is pretty much a must. Not doing anything somewhat worked on 4:3 monitors but it's definitely awkward on 16:9 and on 21:9 your head is definitely on a swivel.

Oh and those large margins are very useful for things like footnotes, btw, or meta-information about the text (like those textbook "this is an exercise" stylings, just move the marking over to the margin). There's also plenty of place for a hierarchical list of contents, always on screen, and various other nav stuff. None of that will degrade loading or runtime performance to any noticable degree.

Also of course note that that's for text-heavy content, stuff you read as in reading an article or book, not stuff you look at in the sense of "reading" a poster. In this case you can e.g. turn those bullet-points into rectangular areas (also come up with a sixth one, then) and display them in a grid, each containing, well, what they contain now but also a link to further information. You see that pattern all over the place on the modern web and it's a good one. Would need quite a bit more content than is present on those websites, though, otherwise you have more navigation shenanigans than content. You don't need a fucking library index for a post-it note.

Source: My HTML is rusty as fuck but I know TeX.

I'm on mobile and the only difference i see is the lines of text on the "better" one are spaced more so I have to scroll farther.

Is it more legible? No, I'm not a fucking donkey and I can read a block of text like a normal person.