Thanks for the feedback.
Zooming doesn't work well in HTML. Probably best you avoid it.
Info boxes are definitely on my "to do" list.
Not sure about the heartbeat issue as I only have my laptop to compare with. It should change pitch, volume and speed as your health deteriorates.
Level 4 has 600 background blood cells compared to 200 on level 1. So I will reduce those which should help with the stuttering