Adapting Text Units: Spaces, paragraphs, and ems

Hello John, Greg, Detlev, Jason, and everyone,

Based on Wayne's research and analysis, I previously suggest the
following for the Adapting text SC.

* line spacing (leading) to at least 1.5 (space-and-a-half)
* spacing between paragraphs to at least 2 (two spaces) larger than
the line spacing
* letter spacing (tracking) to at least 0.12 em
* word spacing to at least 0.16 em

On the call yesterday [1], some folks brought up that they felt the
use of "spaces" to measure leading for the Adapting Text SC is
untestable. At one point line-height was suggested.

Detlev, you were absolutely correct when you said, "if memory serves
line-height needs no unit". In CSS line-height is unitless. Eric Meyer
explained that in "Unitless line-heights". [2] It inherits from the
font size. This was discussed on in the Adapting text GitHub Issue
last March [3].

Moreover, WCAG 2.0 1.4.8 Visual Presentation [4] uses the "spaces" and
"paragraphs" for units. It states:

"...Line spacing (leading) is at least space-and-a-half within
paragraphs, and paragraph spacing is at least 1.5 times larger than
the line spacing..."

Is that currently untestable? If not, why not re-purpose that terminology?

As for using ems as a unit perhaps we need a definition because that
is a new concept for WCAG. How about something like:

em: A length unit relative to the font-size that is used. e.g., letter
spacing of 0.12 em means 0.12 spacing in addition to the default space
between letters of the font size in use.

Ideas for improvement? Other thoughts?

Thanks everyone.

Kindest regards,
Laura

[1] https://www.w3.org/2017/07/11-ag-minutes.html#item02
[2] http://meyerweb.com/eric/thoughts/2006/02/08/unitless-line-heights/
[3] https://github.com/w3c/wcag21/issues/78#issuecomment-288755938
[4] http://www.w3.org/TR/WCAG20#visual-audio-contrast-visual-presentation

-- 
Laura L. Carlson