Output-only capabilities

[gsnedders]

Split from discussion on w3c/webdriver-bidi#446 (c.f. #1790 (comment)).

Not everything in https://w3c.github.io/webdriver/#capabilities has defined matching behaviour under https://w3c.github.io/webdriver/#dfn-matching-capabilities.

If I pass setWindowRect as an alwaysMatch capability, what should behaviour be?

Currently, under "For each name and value corresponding to capability’s own properties:", this falls through out of the "Otherwise" clause because:

• "If name is the name of an additional WebDriver capability" doesn't apply because an additional WebDriver capability is one defined in another spec, and setWindowRect is defined in this spec

• "Otherwise, if name is the key of an extension capability" also obviously doesn't apply

This effectively means that we end up not doing any matching and we also don't fail as a result, and the returned value may well be different to the alwaysMatch value.

From a protocol point-of-view, it is exceptionally surprising that we end up with the returned value being different to the alwaysMatch value.

(Note I'm not super interested in setWindowRect specifically here—and indeed this is arguably a bug—but we do want to allow for other output-only capabilities, and we should define in general what should happen when they are passed in as a capability.)

[whimboo]

CC @jgraham