bug#36304: 27.0.50; request: switch to the superior HTML #RGB convention for colors

[Pip Cet <pipcet@gmail.com>]

This is a request for an incompatible change, which I think will
improve some matters significantly:

The Web convention, used by HTML and SVG, is that #fff is parsed as
#ffffff, and represents "white". The X convention, discouraged for
years, is to accept #fff as shorthand for #f0f0f0, and #ffffff as
shorthand for #ff00ff00ff00, with no way of specifying the whitest
color as six (or three) hex digits.

We're still using the X convention. It's time to switch. The HTML
convention is what virtually everyone would expect these days, it
allows specifying "white" concisely, and it's what we use in some
contexts, such as when rendering SVGs.

Not fixing this would result in somewhat hard-to-track-down color bugs
all over the place, particularly when combining images and text or
using non-X window systems or non-8-bit color depths (again, "#ffffff"
isn't the same as "white" on those!)

There's no reason, as far as I can see, to burden everyone with a
deficiency of one specific graphics backend, in perpetuity,
particularly since the colors will stay similar but not quite the
same.

This issue was discussed years ago as bug #8402, so feel free to mark
this as a duplicate if it helps.