Alter the content as little as possible: This is another good one. If you’re altering the html, preferably you should just change an ID or a class. The more you change, the more chance of mucking things up!
Document output, parameters, and dependencies Basically, comment your code. And build APIs that solve specific problems, rather than multi-purpose APIs that require a potential developer to wade through a lot of irrelevant stuff. Nothing new here for anyone who’s been coding for a while, but it’s still good to mention.