I'm not sure what you're asking. The browser is not an OS, so the websites are not supposed to form a system. People using Microsoft's default browser is yet more evidence that people generally stick with the OS defaults, because they are supposed to work.
I wanted to write OS, and couldn’t edit it later on :/
My point was that windows is the most popular OS despite it having a ridiculous amount of “native” looks. So linux not having one is not the reason for its less than ideal desktop uptake.
Looks are not the most important part. Functionality is.
Besides, as bad as Windows’ UI fragmentation is, Linux’s is worse. When opening a Linux application, from Pidgin to Firefox to the control panel to Telegram, there is absolutely no guarantee of what you’ll be getting. Having 7 design languages is bad, but what’s worse is having 0.