HTML5 support in QtWebKit2 is greatly improved. This will be included in Qt 4.8.0 which is available as a release candidate now and should be released soon (i.e. in the next week or two). If would be great to see encapsulator updated to Qt 4.8.0 so we can take advantage of it (at least on Windows. On MeeGo it's using the platform's installed version of Qt).
For more complete information about compiler optimizations, see our Optimization Notice.