[MarkSweep]

I think the reason this is not done is it would leak to the website if the user had previously downloaded the same file from a different website.

[vanviegen]

Sure. And you can do the same thing with a CDN file by timing how long it takes to load. Sub millisecond means cache.

[notriddle]

It's why newer versions of Firefox and Safari use segmented caches. Which, of course, means that loading fonts from a CDN no longer buys you anything at all.

[imajoredinecon]

Chrome as well. (The zdnet article above is about Chrome, which shipped this last year.)