As expected by the tests, using the default values when the capability is not provided by the user.
Created attachment 347011 [details] Patch
Comment on attachment 347011 [details] Patch Informal r+, LGTM.
Comment on attachment 347011 [details] Patch r=me, although I don't know if emptyString() as a default value may confuse some downstream software like Selenium or Grid.
Committed r234837: <https://trac.webkit.org/changeset/234837>
<rdar://problem/43272816>