

The most important difference between 1 and 2 is, IMO, the width limiter. You can actually read the source yourself, it’s extremely simple hand-written HTML & (inline) CSS. max-width:650px;
stops you needing to crane your head. It also has slightly lower contrast, which I’m told is supposedly better for the eyes according to some studies, but personally I don’t really like as much, which is why “Best” is my favourite, since it has a little button to toggle between light mode and dark mode, or between lower and maximum contrast.
I actually really like the black black. And they didn’t use red red (assuming that term is supposed to mean FF0000); it’s quite a dull red, which I find works quite well. I prefer the high contrast mode though, with white white on black black, rather than slightly lower-contrast light grey text. I’m told it’s apparently evidence-based to use the lower-contrast version, but it doesn’t appeal to me.
Though I will say I intensely dislike the use of underline styling on “WRONG”. Underline, on the web, has universally come to be a signal of a hyperlink, and should almost never be used otherwise. It also uses some much nicer colours for both unclicked and visited hyperlinks.