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Online and free and as simple and as powerful as Paint.NET: https://www.photopea.com/

Photopea is so good it's one of the few sites I allow ads via ublock.

Try the pwa version as well. Feels like having photoshop for free.

Why don't you use paint.net? What's stopping web developers from using it?

I haven't used GIMP on windows for a while, but I see no problems with the interface on Linux.

I use Pinta which has been updated recently: