Lion introduced a new feature called look up:
... which can be invoked by double-tapping a word with three fingers. This works fine most of the time, but I've noticed that it fails many times in Google Chrome (I'm running 18.0 beta).
For example, it works fine on this website, but fails on http://www.gigaom.com. If I try both websites on Safari, they both work. Why does Chrome have this problem? Is it an issue with the website owner not following standards? Is it a bug in Chrome? Is there any temporary workaround?