JAB_Creations - 7:03 am on Aug 29, 2010 (gmt 0)
From what I can tell Microsoft was the first to start working with 3D acceleration and I suspect that they were already a few months in to working on IE9 before IE8 went RTM. I know that other browser vendors did not delay in trying to add support for 3D acceleration.
I don't know much about how software is written though I know there are a lot of sub-systems in browsers that can effect of kinds of performance. In example while Opera 10.6 is the fastest at the SunSpider benchmark it takes a backseat to Firefox 0.7~3.6 (I am not joking about those version numbers) on my (currently private) benchmark that tests what I've been told is a wide array of subsystems. I've seen the first build of Opera 10.5 take Opera from a total loss to running half of the benchmark and varying degrees of increased performance through to the latest 10.7 builds. Firefox 4 goofed something up and the whole benchmark fails. I once took a look at the source code for Firefox...it's simply overwhelming. While IE9 is hardware accelerated it still utterly fails my benchmark though then again so do Chrome and Safari.
How much a browser can take advantage of a GPU will be dependent on many things. An example in gaming is the GPU's frame buffer; the more GDDR memory the greater the radius surrounding the player that various levels of detail can be displayed. Just an out-of-thin-air example let's say that 512 MB could emulate high details up to 150 feet from where your character is standing and 1,024 MB could emulate up to 240 feet away, somewhere the software has been programmed to determine when objects should be rendered in the GPU and browsers will have their respective share of determining the capability of the resources available...if they do actually end up implementing such detections and in a lot of ways browsers do not.
At one point CSS2's positioning properties were cutting edge and at some point 3D acceleration will be common place. It's hardly the end-all and it will come down to browser software having numerous performance optimizations for an exhaustive list of benchmarkable tests. Large steps are still ultimately steps. ;)