Windows has only been like that recently. The 9x days, and even a good chunk of the XP days meant standard looking applications.
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.
It's funny, back in the day it was "It will never be the year of Linux on the desktop until all applications look consistent". People hated the fact that this app was Gtk, that one was Qt and this one here was Motif. But on Windows, it was winapi button windows and common controls all the way (except a few MS apps like Encarta - leading the way in modern flat guis already back in the Windows 3.1 days). The world has turned so far around!
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.