Probably use this

autocmd FileType vim  call RunHtml()

However, it is recommended to set makeprg when autocmd FileType is used, and then directly make when F5 is used. This displays an error message to quickfix.
This is for other languages I have set up.

augroup make_autocmd
 autocmd Filetype javascript setlocal makeprg=jsl\ -nologo\ -nofilelisting\ -nosummary\ -nocontext\ -conf\ /etc/jsl.conf\ -process\ %
 autocmd FileType json setlocal makeprg=
 autocmd FileType php
 \ setlocal makeprg=php\ -l\ -n\ -d\ html_errors=off\ % |
 \ setlocal errorformat=%m\ in\ %f\ on\ line\ %l
 autocmd BufWritePost * call Make()
 " auto close quickfix if it is the last window
 autocmd WinEnter * if winnr('$') == 1 && getbufvar(winbufnr(winnr()), "&buftype") == "quickfix" | quit | endif
 augroup END
 
 function!  Make()
 if &modified | silent write | endif
 if &makeprg == 'make' | return | endif
 let regname = '"~'
 let old_pos = getpos('.')
 silent make
 execute 'cw'
 if !  has('gui_running') | redraw!  | end
 call setpos('.', old_pos)
 endfunction