Or at the very least, if you do have some content for which you absolutely can't yet find a workable pure-HTML delivery mechanism, try to isolate it and provide a seamless fall-back to render a close approximation of that content (or censor it out so that it doesn't appear that anything's missing) in web browsers which do not support a Flash / Silverlight player.
Try to make sure that the absence of any content for which you have no choice but to render in Flash / Silverlight, wouldn't interfere with the ability to access the core functionality of the web page.
If some of the content which would be missing is unavoidably linked to the core functionality of the web page, then you'll have few choices (at present) but to resort to building native Apps, as an alternative to the web page, for each of the affected platforms. (Or else ignore those potential users entirely.)