No offense, but if you use innerHTML the cursor location isn't saved, and if you don't use += or = innerHTML + "<u></u>" all content will be replaced. This will just anger users.

I would suggest reading this tutorial and adapting it to your needs. This is the same thing I used for my work but I got a bit fancy with it; however, this is a great place to start from.

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Programmers are in a race with the Universe to create bigger and better idiot-proof programs, while the Universe is trying to create bigger and better idiots. So far the Universe is winning. - Rich Cook