The View Definition Language, alias View Template Language, alias .xthml is an abstract XML notation that defines a view independent of the destination media. JSF renders Views by taking the compiled Component tree of the View and passing it through a renderer. Renderers are actually plug-in subsystems, however by far the most commonly-used one (and the one that the Facelets subsystem favors) is the HTML renderer.
The browser doesn't understand VTL. It only understands HTML, but that's OK, since that's what JSF's HTML renderer sends back to it. In the event that a View Definition contains unknown tags, it's the decision of the JSF renderer as to what to do with it.
Incidentally, that's why I don't encourage raw HTML on a View template. If you aren't using the HTML renderer, but instead are using some other sort of renderer, such as a PDF renderer, the HTML tags will typically come out as noise in the resulting view because JSF treats them as data instead of JSF information.
An IDE is no substitute for an Intelligent Developer.