[css-3 colors] Transparent backgrounds

I was wondering how a transparent background should be rendered bu user
agents. Suppose the body element (and perhaps the html element too) had a
semi-transparent value - should they be cast against a white background, or
should the window actually appear transparent as well, allowing the
desktop/other windows to show though?

If the latter, how should this be rendered on mobile platforms?

Personally, I would be expecting a white background, but it would be
interesting to have translucent parts of the webpage.

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