[googlefonts] new check: mandatory avar table

In this twitter conversation @davelab6 wished for a fontbakery check on the googlefonts profile to make avar table mandatory: https://twitter.com/davelab6/status/1331698384743387138

[felipesanches]

Current rationale text for this check starts with "Most variable fonts should include an avar table to correctly define axes progression rates."

At https://github.com/justvanrossum/nabla/issues/25#issuecomment-1215650287 @davelab6 said:

@justvanrossum yes, that’s fair, since the description is “most” not “all” then FAIL is heavy handed.

So, I will make this a WARN.

[tiroj]

You could go one step further and a) check this for the wght axis only, b) report a warning if the wght axis covers three steps of the CSS scale (e.g. Adam’s 700–900 example), and c) report a fail if the wght axis covers more than three steps of the CSS scale.

Really, though, this is a design issue, and the question is not whether a variable font should contain an avar table but whether the weight progression should be linear or non-linear. The answer to that question from type designers making fonts with extensive weight ranges is most often that weight progression should be non-linear, and ergo such fonts are going to need an avar table. The greater the weight range, the more likelihood that a non-linear wght progression is desirable, and hence my suggestion to tweak the test accordingly.

The bigger problem here is that FontBakery doesn’t know anything about design intent, so will inevitably produce some reports and recommendations that do not apply to specific fonts. At present, these need to be negotiated between the designers and Google Fonts, and sometimes exceptions are granted, but then those exceptions need to be tracked. One solution to this would be to front-end some information about the design intent as input to FB, e.g. some kind of configuration file to aid FB with information about the font from the font maker such as ‘wght progression is non-linear’ ‘TRUE/FALSE’.