It's great to see more variable fonts being made. One thing I'm wondering about, though, is how variable "italic" is being handled: I see fonts with an "italic" axis where the design variation is really slant, and fonts that have an unexpected scale (for either italic or slant), like 0 to 100, or 100 to 1000.
Is this an issue in tools—that the tools aren't guiding font developers to use the most appropriate axis with the most appropriate scale? Or is it that font developers aren't sufficiently familiar with the registered axes and how they're defined? Or are there motivated reasons for font developers to do things the way they are in spite of how the registered 'ital' axis is defined?
Personally, I think we need to stress more strongly that the ital axis primarily exists to complete the STAT table style/axis mapping model, and that actually including ital as a variation axis in a font isn't something that is generally useful.
With regard to the scale, there may indeed be a tool related issue. If a font does contain a variation along an ital axis—whether correctly interpreted or not—, the visualising this within the UI of the source format will probably require a scale other than 0–1 (the variable font UIs tend to employ integer values). So either this scale needs to be redefined to 0–1 before generating the font, or an avar table needs to be used to map the source scale to the defined ital scale.
Mostly the first two.
BUT, Slant does have the issue that for most fonts using it, if they use the range values the spec would require (from, say -10 to zero), in most apps they get an interface that works in the reverse direction of what users would expect. It just feels *wrong* to move a slider to the left to slant something to the right. I have been working with this for quite a while now, and it's something that I am just not getting over.
In the meantime, for those apps that are smart enough to know the Slant axis as a specific thing, and display axis interfaces with sliders or dials that go the “wrong way,” I suggest they reverse the directionality of that slider or dial.