Modify mode's font-locking without changing source code

Question

Suppose I want to make additions to a major mode's syntax highlighting, how would I do this without modifying the major mode's source code?

Some ideas:

• Redefine variable in .emacs
• Define a minor mode with the changes

Answer 1

Use font-lock-add-keywords. See the Elisp manual, node Customizing Keywords.

The Elisp manual is your friend: C-h i, then choose it. In the manual, i is your friend. E.g., i font-lock TAB.