ReadWriteWeb published the results of a study that compared HTML5 against Flash in a series of tests on both the Mac and PC and in different web browsers including IE8, Firefox, Safari and Chrome. The test found that Flash isn't always a CPU hog, in some cases HTML5 does even worse.
In analyzing the results of the tests, Ozer determined that the key to better Flash performance was dependent upon whether or not it could access hardware acceleration. This feature, launched in Flash 10.1, allows the plugin to use the graphics processing unit (GPU) on some computers to decode video. Depending on the video card and drivers, (NVIDIA, AMD/ATI and Intel offer products that support this), the video decoding process in Flash 10.1 can now work for all video playback, not just full-screen playback as was available in Flash 10.0.