When loading webpages, one of the most important factors to saving power is reaching an idle state as quickly as possible. For many pages, that requires being fast at executing JS that runs only once or only a few times. Safari dominates on this, thanks to WebKit and JavaScriptCore. You can see this on JSBench, which runs JS modeled on real page loads and interactions: http://plg.uwaterloo.ca/~dynjs/jsbench/

I applaud Chrome's recent work on battery life. But I think it would be fair to say that safari is still the best in this area. And also that our excellent results have inspired Chrome and others to do better, much as in an earlier era Chrome inspired other browsers to get more serious about JS performance.