HTML rendering collapses whitespace; it has nothing to do with accessibility. I would like to see the research on double-spacing causing rivers, because I’ve only ever noticed them in justified text where I would expect the renderer to be inserting extra space after a full stop compared between words within sentence anyway.

I’ve seen a lot of dubious legibility claims when it comes to typography including:

1. serif is more legible
2. sans-serif is more legible
3. comic sans is more legible for people with dyslexia

and so on.