People want to style things to match their page, exactly because consistency is part of usability. Especially given the limitations of browser form validations, you will absolutely need your own validations in addition to any browser validations you use. But your own validations will look different from the browser-provided ones, at least on some browsers. Which will confuse users, hence decreasing usability.
And this also assumes that the browser validations are in any way usable to begin with. I would argue that they are not, and would require some sane styling to become usable in the first place.
I think consistency within an application is far more important than consistency across applications. And even if it were true that users hated the lack of consistency, it would fall on deaf ears: the web page developers can only influence their own page. They can't make Google or Mozilla or Apple come up with an actually usable error model, or any kind of good UI in general, so the only chance they have is to create a good UI for their own page, and hide the horrible defaults that each browser is reinventing.
And this also assumes that the browser validations are in any way usable to begin with. I would argue that they are not, and would require some sane styling to become usable in the first place.