Comment on: We finally learned to center a div, then browsers added sidebars
As a rule, you should not add functionality to a website to compensate for browser misfeatures. People who want the browser to work differently can get a different browser: that's why we call them "user agents". Making the web all complicated like this mainly serves to make it harder for people to use alternative browsers, or for browsers to change their misfeatures without breaking compatibility.
Comment on: We finally learned to center a div, then browsers added sidebars
So this interpret the sidebar as a temporary UI element (like a popover window), but for me I'd rather interpret the sidebar as a constant element (like the sidebar of Windows Explorer). And indeed I'm using Firefox with Tree Style Tabs or Sidebery to have an explorer-like tree view of my browsing session; it makes more sense for me to have this as a permanent fixture.With that in mind, I would think this kind of preference should be under the control of the browser, not of an individual website.