What to use for a lightweight GUI

[NovaSquirrel]

I'd like to make a desktop app version of a web app I created (Tilemap Town), because I noticed that some weaker computers like the Raspberry Pi Zero have trouble running web browsers. That means I probably need to pick a widget toolkit or other library, because I'd rather not try to make a GUI directly on top of SDL again. I think most of the advice I find online about what widget toolkit to use points people towards Qt; is that still a good option when targeting the kind of computers I have in mind? I haven't done much with different toolkits and I don't know how they really compare as far as resource usage goes. Bonus points if the toolkit is compatible with Windows 98 or XP, though I'm probably better off just directly writing a separate program that uses the Win32 API if I want to support that.