That can happen. Or users may not even have it installed. But contrast with the case of a user with a browser that doesn't support some features of HTML5. How do you detect which features work for them, and which don't? How can the user fix the problem?

The only way to fix it is to install a new browser, or perhaps a new version of the current browser. Installing a plugin is often easier. Rather than deal with this, developers currently just develop for and test on a few main browsers. Anyone not on these are left with no other options but to switch browsers.

Suffice to say, HTML5 doesn't really improve the situation all that much. It simply trades the closed model that Adobe uses with Flash for an open model that has been used for HTML. In either case, the only way to be really compatible is to use older well established features.

Open platforms are good, and thus HTML5 is a good development. But it's not the silver bullet that many make it out to be.