Guidelines for associating tests with features

In https://github.com/web-platform-tests/rfcs/blob/master/rfcs/web_features.md we introduced the ability to associate tests with features using WEB_FEATURES.yml files, and showing them on wpt.fyi like this:

• https://wpt.fyi/results/?label=master&label=experimental&aligned&q=feature%3Acanvas-text-baselines
• https://wpt.fyi/results/?label=master&label=experimental&aligned&q=feature%3Afont-palette
• https://wpt.fyi/results/?label=master&label=experimental&aligned&q=feature%3Anesting

There are now over 150 features mapped using WEB_FEATURES.yml files. To allow more people to contribute to this, it's clear that some guidelines will be needed.

Questions and tentative answers:

• When should a test be linked to a feature?
  • When it's in the scope of the feature in web-features judging by its description and compat_features list.
  • Tests should match the spec, but contributors aren't expected to verify this, assume the initial review covered this. If it becomes known that a test doesn't match the spec, the best course of action is probably to make it tentative.
• Can a test be linked to multiple features?
  • Yes! But this is only useful when two features ship in different orders in different browsers, to allow filtering out tests that cover a yet-to-be-implemented feature.
• How can a specific test be excluded?
  • There's no syntax for this, but the need is obvious. Right now the only way is to list all tests and remove some.
• What about tentative tests?
  • It's fine if they're captured by the patterns, they can be excluded using !is:tentative on wpt.fyi if desired.
• What about subtests?
  • There's no way to associate only a few subtests with a feature. This hasn't been a big problem in practice, but in a few cases this makes it impossible to capture all the tests.

Beyond this, some form of test suite review by feature experts would be useful, but I'll file a separate issue about that when I've thought through which repo it belongs in.

cc @jcscottiii

Start by reading rfcs/web_features.md and reviewing existing WEB_FEATURES.yml files, then use the questions and tentative answers in this issue to identify the guidance that is still missing. Done means adding agreed guidelines for associating tests with features, including multiple features, exclusions, tentative tests, and subtests.