I am ultrawide monitor owner but use it in vertical mode almost always.
>Do they just stretch all the way or at which max width do websites stop? Are most websites centred or left aligned when cropped?
They have tiny amount of content centered and huge spaces of nothing on right and left. Some Russian websites has a fashion called "rubber layout" so sometimes text might be streched across the width which makes the content very unreadable. BTW using browser on horisontal ultrawide fullscreen is kind of wrong, IMO better to place several different windows across the width. So, if using several different windows, rubber is better, but if using browser in fullscreen - tiny amount of centered content might be better.
> If owners of ultra wide screens could post some screenshots of some websites that would be awesome. I'm interested to see what Twitter looks like, StackOverflow, the BBC website, Reddit and even HackerNews.
Ping me after several days if nobody will answer. I can switch to horisontal, but it takes a little of time to do that screenshots and to reswitch.