Live Editor
Live Editor is an simple in-browser HTML editor that allows you to write HTML, CSS, and JavaScript in realtime and preview the results on the same page.
JavaScript inspired by https://github.com/xem/miniCodeEditor
Demo: https://tomhodgins.github.io/liveeditor/index.html
If you are looking for a more advanced live HTML editor, please check out: https://github.com/tomhodgins/tinkerpad