Sounds awful. No accessibility to screen readers. It's raster-based so you can't zoom. No text selection or copying. And that's just what I thought of off the top of my head...
There is one reason I can see for using canvas based rendering generally[1] and that is if one really really wants to avoid someone from copy-pasting or indexing the page easily[2].
[1]: valid use cases for using canvas exists.
[2]: as someone who has digitized a couple of documents I'd say the effort is just above copy paste (with my tools it is copy, paste, OCR to clipbord, paste, verify)
Shouldn't screen readers use OCR? I mean OCR is basically a solved problem, and there are situations when you want to read what's in an image, or when the HTML is a mess.
Yeah, you'll probably get the best experience if the web developer specifically designs for accessibility. I guess the problem is that it doesn't happen as much as we'd like.