What worries me is the testability. Too many variables, especially external unpredictable ones injected by browsers and user stylesheets, means accessibility testing combinations explode further than they already are.

Full-page zoom - something I imagine was less common in the past for performance reason - is much more in line with "how the site was made, intended to layout as, and tested for."

It's nice that the HN header bar doesn't grow on font-zoom, but it's hardly a killer feature (edit: and isn't even supported on my iPad where I'd want it). I always full-page zoom and was never bothered by it or even noticed it. But clearly half the web is broken for font zoom. I'd say don't fix it, leave it behind instead. Just like 14px doesn't really mean 14px anymore and that's ok.