The syntax checking plugins have their own dependencies, so have a look at the
syntax_checkers directory to see how they work -- some should work out of the box.
CSS3 Syntax Highlighting
Source git://github.com/mileszs/ack.vim.git Source git://gist.github.com/1229444.git Source https://raw.github.com/gist/1229444/6d07d825fa99a26d2dcc0fd83e9a8b1c78978bfa/statusline-help.vim
It actually downloads the files, so it shouldn't cause Vim to download them every time it's opened:
Automatic updates occur once a week when starting Vim. This will
git pullany repositories and re-download any other files.
The author hasn't written any help documentation yet, but the project's readme file has enough details to get started.