Consider possibly deprecating [attr!=value] #207

There is no rush to do so, but moving forward, I think we will shy away from delving into syntax that deviates from the CSS specification. We've started moving custom pseudo-classes over to have prefixes to avoid future conflicts, and it is possible that one day [attr!=value] could have some meaning in the CSS spec in the future, and it could be different that what we currently do.

IIRC this syntax was borrowed from JQuery, but TBH, it really doesn't add functionality as you can do the same with :not([attr=value]).