Re: TAB when the region is active

[Richard Stallman]

    I agree, this is a natural extension and very useful. (And this is the way
    it works in nxhtml-mode currently.)

This is not something that major modes ought to change.  If we adopt
this for Emacs generally, as _perhaps_ we should, then that mode
should do it too.  Otherwise, when we install it, we should change
nxhtml-mode to turn that off (by default, at least).