A tab is a tab and nothing but a tab. There is no *different* encoding. It's just the way a tab is rendered (w.r.t. the number of spaces) which differs. Plus AFAIK, it has got nothing to do with the OS as such but rather the piece of software used when visually rendering a tab.
I can't imagine that we're the only people who want to be able to change the tab spacing in browsers, so I'm going to continue searching the web for a way to render tabs differently visually while preserving a tab's unique functionality.