Comment by vbezhenar

With the abundance of software around I'm pretty sure that the're plenty of GUI authoring tools. According to quick Googling I can see that there's Adobe Animate which is successor of Adobe Flash and it supports HTML5 output.

I don't think that asking from modern software to work on Pentium 3 is reasonable. Software should work on a reasonable percentage of user computers. Pentium 3 is definitely outside of this definition. You didn't ask for Flash to work on 8086, did you? I'd expect it to be a challenge to even run Google Chrome on Pentium 3.