It's been great to see the excitement and interest in variable fonts, and to see people beginning to experiment and build prototypes. There's one concern I have in this early phase, though: the risk of jumping the gun with stuff not fully ready.
When our teams at Adobe, Apple, Google and Microsoft got together to develop the OpenType Font Variations spec, one thing we set out as a priority was to ensure interoperability: to agree on a specification and ensure that our platforms conformed so that font developers and users could be confident that variable fonts would behave the same on all platforms. When we announced the technology, this spirit of collaboration and desire for interoperability on our part seemed to be something that was greatly appreciated. But I'm worried that, in excitement to jump on board, people may overlook some of the details needed to ensure interoperability.
I've seen some vendors starting to market "variable fonts". Yet, I'm not yet aware of any tool chain that fully supports all of the new formats in OpenType 1.8, and all of the requirements for OT 1.8 variable fonts. Of course, it's possible to craft a file conforming to the new formats with nothing more than a binary editor, though doubt many would go through such a tedious process to build a font.
For example, a required table for OT 1.8 variable fonts is to have a 'STAT' table. But I'm not aware of any font tools that support this table yet. Similarly, the 'MVAR' table would be needed to support any variation in font metrics, but I'm not yet aware of tools that support that either.
With these things in mind, I thought it would be useful to start a discussion thread focusing on the readiness of tools that fully support the variable font spec. I invite different tool vendors to provide occasional updates regarding what parts of the OT 1.8 spec their tools do or don't yet support.