Recently I discovered it's possible to have syntax-highlighting in a <textarea> using JavaScript.
Are there Open Source libraries which also support auto completion? I'd like to make a simple online editor for HTML/CSS templates.
Preferably, the completion is can be extended, so I can add custom rules.
You should check this wikipedia page on javascript based code editors. It lists the MDK-Editor as one that supports auto-completion for html, css and javascript. You can see a demo here.