It's difficult to be very precise in answering this question. For desktops and laptops, the latest statistics I've seen show more than 20% of users still rely on versions of Internet Explorer that don't natively support new HTML5 features, but well over 90% have the Flash plug-in installed. But then you need to pay some attention to cell phones and tablets, where you'll find nearly universal support for HTML 5, but none of the Apple devices support Flash. So it all depends on your audience as to what the final mix is for a given website.
Rick Trethewey
Rainbo Design
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