ignisferroque: Computer related:
• Most modern computer screens being only 16:9. I like my 16:10 ones, Id like to replace them with 16:10 ones one day, but it doesn't seem very likely.
I would rather have 4:3 monitors, as I find the height of "widescreen" monitors to be too limiting, and the width too much, but it seems they don't make them anymore for whatever reason.
Related: Websites which have headers or footers (sometimes both!) that stay in place instead of moving away when you scroll, reducing the amount of visible space available.
JavaScript: Both the fact that many sites won't work without it when they would be expected to (for example, sites with text content like articles), and the fact that JavaScript in webpages is overused and quite bloated, making much of the web unusable on low powered or older devices. (Also, from what I hear JavaScript isn't that great of a language, to the point where people have written programs to translate other languages to JavaScript, and even invented other languages for this purpose (like TypeScript).) There's also the fact that some tasks are really better done server-side; for example, if serving web pages that only change on occasion (the GOG home page would be an example here; except for things like timers, the page only changes when news is added or sales end/start), the page can be generated once per change instead of requiring the client to execute JavaScript and make more network connections.
Auto-playing videos on websites: This can be a problem when a link is clicked accidentally, and even if not, I might want to read the description or make the video full-screen before I start it. Also, animations are distracting. (YouTube and Twitch are offenders here, as are many news sites.)
Browser discrimination: Facebook does this. Try to access the site with a browser that doesn't pretend to be one of the major ones, and you will be redirected to an "unsupported browser" page. Instagram also does this, preventing you from posting anything unless you are using a mobile device (or have configured your browser to like about this). Also, many other sites give me 403 or sometimes even 503 errors when I am browsing with Lynx. This is *not* what the "User Agent" string is for. (One note: Facebook fixing this issue still wouldn't make the site acceptable for me.)
Assumption that everyone has a smartphone: I have seen this in a couple places. One, a tech conference where there are no printed copies of the schedule available. Two, a mall which (as far as I can tell) has no directories, so I can't see what stores there are.
Phone numbers as required fields: I really don't like giving out my phone number, but many website forms (including some for registering for in-person events, like a Yoga class I attended recently) require it as a mandatory field. Also, doing this assumes everyone who can access the site has a phone number, and that is not always a valid assumption.