This looks great! Weirdly I just started printing out dice from Thingiverse and was wondering if there was a tool just like this. The selection of icons on game-icons.net is great, but I'd be particularly keen to have the option to design my own - it's great to see you're already considering the possibility of custom ones. It would also be nice to have a range of D6 models, even just a plain cube.
Having played around a bit with this, I notice that "flip normals" sometimes changes the icons from positive to negative (if that's the right terminology - they end up sunken into the face of the die rather than standing out from it), and other times mirrors them. I'm not sure if it's down to the icon or the face it's on or if I've somehow screwed up and am toggling between two entirely different settings. In case it makes any difference, I downloaded this and launched it through the itch.io app - I realise an actual web browser may be more reliable.
Spectacular work here. I've only really had the quickest possible play around but as far as I can tell my throwaway design looks fairly printable in Cura, which seems like a pretty good indication that just about anyone could pick up and use this. The fact it's HTML5 makes that even better. If I actually print a design made in this, I'll be sure to post a photo.