64-bit IE is hardly a common browser (probably well below 1%). Not that I would use silverlight in the first place as not enough people have it installed (nor is its future certain). And the percentage of browsers that properly supports a decent subset of the HTML 5 features is still WAY too low to be "viewable universally" like you want. Flash has a good install base but it's not supported by a some devices, like the iphone/ipod/ipad and a bunch of other mobile devices (including a lot of android devices). If you can live without those gadgets, then HTML 4 + Flash is still an option. Where does this tidbit come from? I just did a test, deployed Win7 Pro x64, and installed IE9 x64 with Silverlight. My test page was http://www.mahorney.net and that site works fine. I did have to install a newer version of Silverlight for it to work, however.