Syntastic

Syntastic (vim.org: Syntastic) by Martin Grenfell is a syntax checking plugin. Syntax can be checked on demand, or when a file is saved and opened.

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

CSS3-syntax-file-for-vim (vim.org: css3 syntax file addon) by "kight" is a syntax file for CSS3. It supports vendor prefixes, and various CSS3 properties like border-radius and box-shadow.

Source

Source (vim.org: source.vim, License: Vim) by Jon Suderman can open remote files or plugins in a similar way to source:

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 pull any 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.