In this case, "hardware accelerated" means drawing web content into an offscreen image -- as the content is transformed using CSS, the image is re-drawn and ble

In this case, "hardware accelerated" means drawing web content into an offscreen image -- as the content is transformed using CSS, the image is re-drawn and blended using the GPU. This is faster than re-laying-out and re-drawing the web content directly (even if re-drawing the web content itself uses the GPU). It's a good trade-off when you have animations that update every frame. The biggest cost is memory. Every pixel of hardware accelerated web content gets 4 bytes of memory. This adds up quickly, especially since any content that overlaps hardware accelerated content must become hardware accelerated as well. The other big cost, as olsn already pointed out, is that blending these offscreen images onto the screen takes time and power on every frame. In extreme cases this can cause scrolling to stutter. See the "Optimizing Web Content in UIWebViews and Websites on iOS" session from WWDC 2012 for more information about this.