Browser based IDE - Textarea with code completion available?

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 开发者_JS百科templates.

Preferably, the completion is can be extended, so I can add custom rules.

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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.

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CodeMirror has ability to support autocompletion

• JS example: http://codemirror.net/demo/complete.html
• XML example http://codemirror.net/demo/xmlcomplete.html

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Mozilla Labs has Skywriter (This project is no longer active) which is a browser based IDE. I don't know if it has code completion (I couldn't find it) though.