Default style

The default background is rgba(0,0,0,0.8).  I've never seen a modern
captioning use a dark background--it's painfully ugly.  Most subtitles use
a stroke for visual contrast, eg. white text with a black outline.    (Some
use a simple shadow instead of an outline, but this doesn't work as well.)

The spec mentions allowing text-outline.  I'm not sure if that still
exists; in http://www.w3.org/TR/2007/WD-css3-text-20070306/#text-outline but
not http://dev.w3.org/csswg/css3-text/.  That's what's wanted here, but
failing that here's an approximation that is at least much better than a
black background:

text-shadow:
    #000000 -1px 0px 0px, #000000 0px -1px 0px, #000000 0px 1px 0px,
#000000 1px 0px 0px,
    #000000 -1px -1px 0px, #000000 1px -1px 0px, #000000 -1px 1px 0px,
#000000 1px 1px 0px;
}

A real text-outline would be much better, especially at bigger font sizes,
where the shortcomings of this become more visible.  Also, usually a
thicker outline than 1px is used, which this doesn't handle very well in
general, especially with serif fonts.

Either way, let's give nice-looking, reasonable defaults that people might
actually want to use, not something that everyone is going to have to
change.

-- 
Glenn Maynard