Adding to this: without any specification, the browser should, and does, use the operating system's native controls when rendering controls such as <button>.
https://blog.chromium.org/2020/03/updates-to-form-controls-a...
All part of Google's master plan to commoditize desktop OSs. Everyone outside the US already uses Android, soon your computer will be a Chrome box instead of a Windows/Mac/Linux box.
First they did the browser title bar & tab bar integration/styling & removed the normal OS title bar, then they made Chrome OS, now they are removing the last traces of OS-specific styling for those who haven't already switched to Chrome OS.
And Gmail, Gdocs, and Chromebooks are huge in kids schools. The kids don't know anything outside the non-Google universe. Most of the kids I know think MS Word/Excel/PowerPoint are lame (not that I'm a fan, but you have to admit MS Office is unrivaled in its abilities).
20 years from now they will have a gigantic monopoly over a huge range of computing.
I'm hoping Firefox Foundation or something like that will do some work in this area.
And I think that web applications should be like this. They may be delivered differently, but they should be part of the system like everything else. Making the UI elements look like the OS ones is good.
I especially notice it on KDE, since on the default KDE theme the checkboxes and radiobuttons and scrollbars look a bit different from most other OSs and the Gnome default theme. But in the browser my checkboxes and radiobuttons and dropdowns look exactly like every other checkbox and radiobutton and dropdown on my computer, and the scrollbars have the distinctive KDE look too.
Most web apps do behave differently from normal applications, and also have stupid company-specific theming, but it's nice having at least parts of them fit in.
I suspect those who come from a Windows background will not have the same opinion, because on Windows basically every app is a mess of different themes and window decorations.
Now that CSS and Javascript are so widely used, it would be interesting for OS vendors to provide a default OS theme along with their packaged browser, that allowed some kind of sane default for web applications to use. You could use a native skin, or load your own CSS to customise it. Microsoft kind of tried this with ActiveX controls, and then Java Applets tried it, and then Flash was fun by ultimately not a good idea, and now we're here with thousands of themes to choose from and a rough set of guidelines that some follow and many don't.