<div>
<span>
</span>
</div>

={motion}   Filter {motion} lines through the external program
            given with the 'equalprg' option.  When the 'equalprg'
            option is empty (this is the default), use the
            internal formatting function |C-indenting| and
            |'lisp'|.  But when 'indentexpr' is not empty, it will
            be used instead |indent-expression|.  When Vim was
            compiled without internal formatting then the "indent"
            program is used as a last resort.

equalprg=
  indentexpr=HtmlIndent()
    Last set from /usr/local/share/vim/vim74/indent/html.vim

<div>
    <span>
    </span>
</div>

:filetype plugin indent on " enable the filetype indent plugin
:e                         " re-open the file (very important!)
:scriptnames               " make sure you see an ".../indent/html.vim"
:=G                        " or whatever range you want