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Re: VOTE: Changing completions-common-part face's default

From: Yuri Khan
Subject: Re: VOTE: Changing completions-common-part face's default
Date: Thu, 7 Nov 2019 20:27:07 +0700

On Thu, 7 Nov 2019 at 17:28, João Távora <address@hidden> wrote:
> > blue3 is *very*
> > noticeable, which brings back the main objection to using it for
> > prefix completion.
> >
> > I recognize others feel differently, though.
> It seems to be more than "feeling".  Have you asked yourself why the
> whole industry seems to does it differently (i.e. with bold or some
> other contrasting color)?  I think it's because not all people have your
> spectacular eyesight and not all of them have such great monitors (or
> use them at a more conservative power setting).

I thought I’d chime in with some actual color science to back the
feelings in this thread.

The Web Content Accessibility Guidelines by W3C define a set of
criteria that text on the Web should satisfy in order to be considered
accessible. The complete document is at
<https://www.w3.org/TR/WCAG21>. Among other things, it defines color
luminance and contrast ratio, and specifies that text at normal size
should have a contrast ratio of at least 4.5:1 (and 3:1 for large
size, where large means 14pt bold or 18pt non-bold).


Practically, there are online color contrast calculators such as these:


(but I didn’t check which of them work well with LibreJS).

I also happen to have an implementation of the WCAG formulae in Elisp at:


Here are some values for pairs mentioned here:

black on white, default text in GUI Emacs: 21, passes
blue4 (#00008b) on white: 15.29, passes
blue3 (#0000cd) on white: 11.16, passes
cyan  (#00ffff) on white: 1.25, fails
cyan4 (#008b8b) on white: 4.14, fails by a small margin

There is no formula for calculating how distinguishable two different
text colors on the same background are. Because of color blindness,
there will always be users who cannot distinguish almost any two
colors that you pick. I think the relevant guideline is 1.4.1 Use of

    Color is not used as the only visual means of conveying information,
    indicating an action, prompting a response,
    or distinguishing a visual element.

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