The CSS3 Fonts spec is still under development but I think it's useful to begin work on the test-suite for it. Some parts of the spec are close to being done and it would help to have an idea of how interoperable different features are across user agents. It would also promote more focused feedback in cases where a given user agent doesn't pass a test.
For the CSS 2.1 test suite we submitted a set of font reftests, I'd like to start with those and expand the tests to cover all CSS3 Fonts features. Support for OpenType font features is still likely to go through several more iterations but I don't think it will be too difficult to track those changes.
Although these tests will be in reftest form, I'm going to try and design them such that they'll still be suitable for general manual testing (e.g. FAIL appears on failure, PASS on success). The tests will include a set of test fonts that will need to be installed locally.