Visibly hidden controls

Hi All,

I'm trying to make a case in opposition of visibly hidden controls. Are
there any WCAG 2.0 success criteria that controls to be visible without the
user having to work hard to discover them?

Chromeless media players are a common example. By default there are no
controls other than (perhaps) a large play button that's overlaid over a
poster image. When the video is playing there are no controls at all.
However, if a mouse user hovers over the video a control bar appears.

Depending on how it's coded the control bar might be accessible to screen
reader users, and might even be visibly exposed to keyboard users once it
receives focus, but from my perspective if it's not visible, keyboard users
won't necessarily know they can tab to it.  The same applies to
speech-to-text users. How do they know they can say "Click mute" if the
media player has no visible Mute button?

Does WCAG 2.0 offer any support for my position that controls should be
visible? I'm also willing to consider opposing arguments if anyone
disagrees (perhaps a clean, simple interface is more or equally
justifiable).

Thanks,
Terrill

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Terrill Thompson
Technology Accessibility Specialist
DO-IT, Accessible Technology Services
UW Information Technology
University of Washington
tft@uw.edu