Problem is, both Firefox and Chrome look out of place on it when you use a non-standard color scheme. Custom keyboard shortcuts won't work on Chrome. When you change a theme from light to dark and vice versa, both browsers like to have text in their UI stuck on the wrong color.
I could use Konqueror, I guess, but its ad blocking plugin (and plugins overall) seems to never have progressed much since KDE 2.
Right, applications can break all sorts of UI consistency. But the question is: do the standard components that the vendor ships do it? I argue: KDE is consistent. I don't want my desktop UI to dictate how an app draws its UI (or games would be impossible).