As explained in 'The core technology' section of my introductory article
on variable fonts:
Five more-or-less common axes of variation have registered axis tags in the OpenType Font Variations specification — Weight <wght>, Width <wdth>, Optical size <opsz>, Italic <ital>, and Slant <slnt> — and these have some assumed behaviours. Font makers can also define custom axes with their own four-character tags and localisable names stored as strings in the ‘name’ table. Additional common axis tags may be registered in future, if these emerge as font makers engage with the technology.
There are always going to be some custom axes that are very much design-specific, or that apply only to a small set of fonts. Rather than trying to guess which axes might become common, the variable fonts working group decided to register only the initial five, and to watch to see if others emerge, registering these at a later date. I think this is a sensible decision, but now that people have had a chance to engage with the new technology at least mentally, I think it may be useful to try to identify potential registered axes and consider how these might best be specified. Not only will this constitute some advance work on such specification, but it can also help people engage with the data models used in variable fonts.
There are three aspects to registering a design axis that should be considered once an axis is identified as potentially common enough:
- Registration in the 'fvar' table, including axis tag, description, value scale, and 'normal' value.
- Corresponding registration in the 'STAT' table.
- Possible corresponding CSS Fonts Module handling (this, obviously, is external to the OT spec).
Please use this thread to propose and discuss possible candidates for registered variable font axes. I suggest using hashtags at the head of messages to identify a specific axis under discussion.