This can be seen exceptionally when watching videos where some implementations prefer Flash. I often fall asleep to documentaries (maybe not a typical use case) and I've seen massive gains in battery life using Safari (using HTML5 video) vs. Chrome (using Flash). Granted, this isn't a fair comparison as one is using a "native" implementation and one is using Flash. But I've seen something like 250% gain in battery life – which sometimes means the difference between seeing the ending and not.

It's interesting watching one's projected battery life climb from 40m to 1h 40m simply by swapping one application for another. However, it's unfortunate some websites prefer Flash-based implementations over their native counterparts. Although this trend seems to be shifting in favor of those that are native. And I assume the smart folks at Google are taking note. YouTube, for example, has done a particularly spectacular job at adding AirPlay functionality to their HTML5 player.

(Disclaimer: I work for Apple)