bug#21557: 25.0.50; HTML renders text invisible

[Eli Zaretskii]

> Date: Tue, 29 Sep 2015 08:02:17 +0900
> From: Katsumi Yamaoka <yamaoka@jpl.org>
> Cc: rms@gnu.org, 21557@debbugs.gnu.org
> 
> On Mon, 28 Sep 2015 16:14:46 +0300, Eli Zaretskii wrote:
> >> +  (when (and (or fg bg) (>= (display-color-cells) 256))
> 
> > I wonder whether 256 above should be replaced with a much lower
> > number, like 88, perhaps?
> 
> 256 means `Depth 8 Pseudo Color', i.e., 24bit color (R:8bit,
> G:8bit, B:8bit) but only 256 cells in the color pallet, AFAIK.
> It matches shr-color.el that calculates colors based on 24bit.
> But I don't know what is 88-color system.  Is it not based on
> 24bit color?  If not, a color shr-color offers will be too dim.

88-color modes are supported by newer versions of xterm.

My reasoning was that when you have 88 colors to use,
shr-color-visible-luminance-min might do its job well enough for us to
be able to honor the colors requested by HTML.