Feature Detection and User-Agent Sniffing.
Generally, I like the idea of having pre-built caches of feature tests for older, known browsers such as Internet Explorer 6 and 7. We absolutely know the issues with these browsers and neither the issues nor the browsers are going away anytime soon. I’m less convinced of the need to cache information for other classes of browsers, especially those that update with regular frequency. For instance, I think it would be wasteful to do such caching for Chrome, which auto-updates at such a dizzying pace that I can’t tell you off the top of my head which version I’m running on this computer.
Nicholas Zakas on the recent re-surfacing of the feature detection v. user-agent sniffing dilemma.