The Wayback Machine - https://web.archive.org/web/20121106234159/http://aestheticallyloyal.com:80/public/optimize-legibility/

Cross-browser kerning-pairs & ligatures

Improved handling of kerning pairs and ligatures in modern browsers using the text-rendering: optimizeLegibility; declaration.

The declaration is currently supported by:
Safari 5, The Webkit Nightlies & Chrome.

Firefox already uses optimizeLegibility by default for text sizes above 20px.

Paul Irish was so kind as to pass on a few bugs that have surfaced.

In recent versions of Chromium, mixing optimizeLegibility and font-variant: small-caps will cause the small-caps to not render.
Irritatingly, it also causes text to disappear completely on webOS.
Joseph Pearson also points out that on slower machines (such as mobile devices) it can negatively impact page load when applied to large blocks of text.
In Safari 5, when using a measurement type of ex on the margin, padding, border-width or outline-width properties causes the browser to crash. See his demo page here: (warning: this will crash safari, obviously) http://quorning.net/safari_crash.htm

AVAST