I personally like it around 67%.
I'd prefer most sites have smaller fonts; there's no need to complain about it, though, because browsers allow you to make text smaller pretty easily, as long as the site doesn't go out of its way to deny you that.
The important part is not so much getting the right font size, but making sure the user's browser can change it well enough.
I haven’t looked into the best way to write web pages that accommodate zooming, but there is a huge variety in how pages perform as you zoom.
Loads of sites don’t do it properly, where you zoom and all the text and images get squashed in the middle of the page, or where the elements disappear off the side of the page. The other horrid thing is when you zoom and then the text is the right size but you end up having to scroll left to right to read the text.
The best sites perform perfectly at every zoom level, with all the content resizing properly so the reading experience is always pleasant.
I’d love to read some tutorials on how to ensure your website performs in this way. In my opinion it’s one of the signs of a really skill-full developer.
The less you do, the best the web page behaves.
The more you fight against default behaviors / layouts, the more things break.
But more specifically, be careful with margins, they grow with the zoom level, often more than wanted. Same for position. And be careful with sizes fixed using something else than percentages for the same reasons. If you don't want a block be larger than 100ish characters for readability, use max-width with a font related unit, not straight width and not some other kind of unit.
I can read smaller text for a bit in a single block, but when it's something like a comment section I need things a lot bigger to be able to skim through comments to spot things I'm interested in.
HN is great too: an assortment of pieces of text, having varied lengths and organized into a tree-like hierarchy, warrants tighter typography.
If the content tries to do this on behalf of certain users then site X ends up being 1.5*1.5=2.25x zoomed while site Y ends up being 1.5*.66 = 1x zoomed effectively turning a single setting that solved the accessibility and preference problem for everyone into a per content-piece nightmare again. Font size/weight/settings in a design is meant to change relative proportions of a design (e.g. headers, emphasized words, side notes) not to act as the overall scaler for the user - that's already available out of the box.
If this post weren't how browsers lay out web pages I wouldn't be such a stickler but to ignore this is to throw out the main advantage of the sizing model browsers use.
The page's ToC uses 60% of the body font, which is ~10pt.