[css3-speech] voice-balance and azimuth

# The ‘voice-balance’ property manipulates the distribution of audio
# output between left and right channels in stereo-capable sound devices.

So.. I'm a little concerned about this definition, as we can't have
both a 'voice-balance' property /and/ an 'azimuth' property, since
afaict they control essentially the same thing. I think it's fair to
assume that we'll at some point add controls for surround-sound and
design CSS with that in mind, so that we don't paint ourselves into
a corner here.

So, if we wanted to add 'azimuth's capabilities to CSS how would that
work? Do we want to have the current definition as well as <angle>
combined into 'voice-balance'? Revert <number> to <angle> and define
how it maps into 2-channel audio? Something else?

3.2. The ‘voice-balance’ property

# <number>
#
#    An integer or floating point number between ‘-100’ and ‘100’.

<number> is a defined CSS value type; you should not redefine it as
"an integer or floating point number". Try

   | <number>
   |   A value between ''-100'' and ''100'' inclusive.

Also, you need to define what happens if, e.g.
   voice-balance: 120;
is specified: is it clamped to the range, or thrown out as invalid?
(I'm not sure which is a better idea; dbaron would know.)

~fantasai