To complete syntax words, you can use omnicompletion:
setlocal omnifunc=syntaxcomplete#Complete
And then CTRL-X CTRL-O to activate completion.
A more robust solution from :help ft-syntax-omni
if has("autocmd") && exists("+omnifunc") autocmd Filetype * \ if &omnifunc == "" | \ setlocal omnifunc=syntaxcomplete#Complete | \ endif endif
This version will avoid changing omnifunc if it's already set (presumably to something more comprehensive).