I followed the instructions of a tutorial I read and downloaded the following file into ~/.vim/syntax: html5.vim.
However, when I opened up vim I didn't see any of my HTML5 keywords highlighted. I had to type :source ~/.vim/syntax/html5.vim to see the changes take effect. (For what it's worth, I had to create this directory myself - it didn't exist beforehand.)
Why didn't vim automatically detect this file? Do I have to add source /path/to/syntax.vim to .vimrc every time I want to include a new syntax file?
Edit:
Figured out the answer. I moved html5.vim into ~/.vim/after/syntax/html/html5.vim. It now knows to add these changes for html filetypes.