[whatwg] Adding alpha channel to <input type=color>

[Victor Vasiliev]

Section 4.10.4.1.15 says that color must be in sRGB. However, it may be
handful for some applications to have alpha channel specified, since
many GUI libraries (GTK has it; I'm also told Mac OS supports it as
well) support color picker with opacity control.

I suggest to add a boolean attribute "alpha" to <input type=color>
(disabled by default). If this attribute is present, the color well
allows to set opacity value, and instead of sRGB, sends RGBA.

Another question is which format should be used to submit RGBA color. We
have rgba(number, number, number, float) used in canvas, but I feel like
it's not the best format to be used in submited data. Maybe #rrggbbaa or
#aarrggbb?

--vvv