The zoomed-out font-size threshold in WebKit is a mysterious anomaly I’ve long had my suspicions about, but now have finally tested. Take some body copy, start zooming out the page, and observe how copy with smaller font-sizes seem to maintain some legible size, even though they should be much smaller. Try hitting ⌘- too see the effect on this fiddle.

View fiddle - zooming font-size threshold

Here’s how Chrome 16 renders the fiddle with Zoom Out hit twice. Zoomed-out font-sizes 13px-9px are all rendered as the same size. Smaller font-sizes return to proper proportions. Note that this is not Safari’s preference to limit small font-size, as the teeny-tiny font-sizes are still rendered.

Zooming out four times, it appears that the threshold is setting a font-size to 9px. Any text set to 9px font-size or greater will be rendered at least the minimally-legible size of 9px. Anything smaller will be rendered proportionally.