If I complain to Mozilla that they deviate from the W3C spec in some respect, will they treat that as a bug, or will they more likely say "no-one implements this, so no website uses this, so we have no reason to be the first to implement this because we don't think this is part of the spec is really important"?
Depends on the deviance. If it's 'this was changed in the whatwg spec' the answer will probably be 'the next w3c spec will document this change, for now here's <an explanation and/or an update to mdn>'.
If the deviance breaks production apps sometimes Mozilla will roll it back, it's happened before.
In other cases they'll treat it as a bug and fix it.
Although I still worry about the case where fixing the deviance would break production apps. Potentially break them, since people won't complain until you release the breaking browser version. That's what's usually kept browsers from hewing to standards.
