Bugzilla

(In reply to James Graham [:jgraham] from comment #10)
> The [spec](https://w3c.github.io/webdriver/#dfn-matching-capabilities) seems to say that either a) you fail to create a new session or b) you return the set value (see the "otherwise" clause in step 3). I don't think it's clear enough if we should fail the session in case it's an extension capability we don't support.
> 
> If the capability isn't an extension capability we should have failed earlier in [validate capabilities](https://w3c.github.io/webdriver/#dfn-validate-capabilities)

If we consider webSocketUrl, and we say it starts being

(submitted unintentionally when updating the title)