Microsoft posted a detailed explanation about the difference between full and partial hardware acceleration, and why its upcoming IE9 will be the only browser to support full hardware acceleration.
In the first IE9 Platform Preview, hardware acceleration applied to everything on every Web page—text, images, backgrounds, borders, SVG content, HTML5 video and audio—using Windows DirectX graphics APIs. With PP3 in July, IE9 introduced hardware-accelerated HTML5 canvas.
Full vs. Partial Acceleration
With IE9, developers have a fully-hardware accelerated display pipeline that runs from their markup to screen.[...]Windows' DirectX long legacy of powering of the most intensive 3D games has made DirectX the highest performance GPU-based rendering system available.
When you run other browsers that support hardware acceleration, you'll notice that performance on some of the examples from IE Test Drive site is comparable to IE9 yet performance on other examples isn't. The differences reflect the gap between full and partial hardware acceleration. As IE supports new, emerging Web standards, those implementations will also be fully hardware accelerated.
IE9 is the first and only browser to deliver full hardware acceleration of all HTML5 content.