There was a period where there was major demand for form widgets in the browser to have a "native look and feel" to decrease the learning curb for people coming from native apps (while meaningful at the time, is less of an issue when most things are web-deployed), and then like the next month there were demands for CSS styling of form widgets and browsers were being shat on because they only supported it in a limited fashion, only color changes, and not actual shape and text styling.
Now you can make text on a button bold italic, but who knows what that means when you see it in relation to a button that isn't bold italic or is just bold or just italic (other than that the designer has bad taste). I once worked with a designer who wanted control over the focus-caret styling (the dotted line in Firefox/highlighting in Chrome that indicates where keyboard input is destined, which changing has potentially huge impacts for accessibility). Having a UI/UX that grossly visually distinct negatively impacts usability, discoverability, and accessibility, IMO, for very little gain in branding (unless your branding is purposely meant to be "confusingly different from everyone else").
Now you can make text on a button bold italic, but who knows what that means when you see it in relation to a button that isn't bold italic or is just bold or just italic (other than that the designer has bad taste). I once worked with a designer who wanted control over the focus-caret styling (the dotted line in Firefox/highlighting in Chrome that indicates where keyboard input is destined, which changing has potentially huge impacts for accessibility). Having a UI/UX that grossly visually distinct negatively impacts usability, discoverability, and accessibility, IMO, for very little gain in branding (unless your branding is purposely meant to be "confusingly different from everyone else").