Nothing wrong on my Android tablet, page renders just fine here. Fennec is my default browser BTW.

But also, the program is for Windows, Mac and Linux, they don't seem to have a mobile version.

Interesting, what browser and extensions? Looks like it's rendering fine on my end for chrome and Firefox on Android, with or without my normal extensions like Dark Reader.

PixiEditor - FOSS from Poland EU, Mainly for Pixel art and sprites, it's a good app.

A good FOSS image editor is eg. this one

https://viliusle.github.io/miniPaint/