Adobe will host a series of technical meetings about the OpenType Feature File Specification in order to to:
- Enable collaborative discussion absent a formal standards body
- Help move the specification forward
- Reduce fragmentation across implementations
This repository provides a forum for gathering proposals (in the form of PRs to change the specification and feature file grammar) and ideas (in the form of issues) prior to the technical meetings. Although most of the participants in the meetings themselves will be developers of the tools that implement the specification, proposals and ideas from a wider audience are welcome. Keep in mind, though, that proposals along these lines are most likely to be of interest to the reviewers:
- Catching up with syntax changes implemented or proposed in non-AFDKO compilers, and therefore not yet included in the current specification
- Support for variable fonts
- Smaller proposals that will easily fit into the existing system
Adobe has proposed additions to support variable values in feature files that are already included in this draft specification, and will be proposing changes to support feature variations (e.g. axis-controlled substitution). Both will be discussed as part of this process.
We are currently in the first phase of the process when proposals and ideas are being collected. The eventual schedule of meetings is still to be determined, and will depend in part on what is submitted and ongoing communication among potential reviewers. However, we ask that anyone who may wish to submit something either add an issue or PR or contact us at feature-file-workshops@adobe.com by the deadline below.
Important
The deadline for initial proposals and participation requests is August 15th, 2026.
The version of the specification we are starting with was the one reviewed last
year in
adobe-type-tools/feature_file_change_review
The most significant change was adding support for variable values in feature
files, but the grammar has also been updated to add a number of OS/2 and hhea
fields that were simply absent from the AFDKO grammar: the subscript and
superscript size and offset fields, StrikeoutSize, StrikeoutPosition,
CaretSlopeRise, and CaretSlopeRun.
Our starting from that point does reflect our belief that those changes are well motivated and structured. However, this should not be seen as our taking further changes "off the table". We recognize that these parts of the grammar can be modified more easily than other parts given how recently they were introduced.
Anyone can make a proposal to add to or change the feature file specification by submitting a PR to this repository. It is strongly preferred that proposals include the following, and that all changes follow the style guide:
- An explanation, in the PR summary, of the purpose of the changes and the expected implementation in feature file compilers.
- Edits to the specification itself
- Edits to the Antlr4 tokenizer and parser files encompassing the proposed changes to the feature file grammar.
- Example feature files making use of the grammar, with comments.
Changes to the variable feature guide, when relevant, are also appreciated.
Submitters should be prepared to respond to questions about their proposals and to make changes if requested.
If you have an idea for how the feature file specification might be changed, or want to raise a concern about something the current specification does not support or only supports in a limited or inconvenient way, and are not able to develop the idea into a full proposal, you can communicate the idea by opening an issue in this respository.
There are no particular guidelines for issues beyond the usual plea not to waste other participants' time. For any idea to have an effect on the specification it will need to be developed into a proposal in some form, presumably by some other participant. Focused ideas about clear needs are the most likely to garner the right kind of interest.
Much of the value of the OpenType Feature File Specification comes from it being supported in multiple font tools stacks and contexts, so that font developers can switch between different tools with minimal effort. This value rests on the common interest of the developers of such tools—functionality added to the specification but not implemented in compilers is of little benefit. Accordingly, we hope that the primary participants in the technical meetings will be developers of tools that implement the feature file grammar.
If you are a tool developer who would like to participate please contact us at feature-file-workshops@adobe.com telling us a bit about the tool, the extent you think you will be able to participate, and the github usernames of potential attendees. We will work with you to arrive at a schedule in the later stages of this process, and may also direct you to specific issues, PRs, or discussions in this repository. We expect to create a schedule issue later in the process that participants can watch to get updates and comment on to raise concerns.
In addition to tool developers, we think it would be valuable to have some experienced font engineers participate in the meetings. If you are a sophisticated user of feature files in font development, or are knowledgable about an area in which there is a significant proposal, please also reach out to us at feature-file-workshops@adobe.com
If you have some concern that does not fall neatly into any of these categories, and have reviewed the other sections of this repository for potential answers, you can also create a new discussion.
Please remember that the specification itself is available under the Apache License, and any additions or changes to the copy Adobe maintains will continue to available under this license. Accordingly, inclusion of patented technology or other features with incompatible intellectual property rights is out of scope of these workshops.
In some initial discussion about these workshops, other developers of font tools expressed interest in better communication across the community. While that need is mostly beyond the scope of this repository, we are hosting a discussion on the subject.
- OpenTypeFeatureFileSpecification.md — Current specification (baseline)
- grammar/ — Antlr4 grammar files from AFDKO
- examples/ — Example feature files
- Variable_Feature_Guide.md — Guide to variable feature file syntax
- STYLE_GUIDE.md — Guidelines for PRs
- TOOLS.md — Notes on the specification preview and grammar checking tools in this repository
- LICENSE.md — Apache License 2.0
If you have questions not answered here you can contact feature-file-workshops@adobe.com
Copyright 2025 Adobe. Licensed under the Apache License, Version 2.0. See LICENSE file for details.