RoToRa's right, except you need to use TRUE, not yes.
The below code works. You will have to style it more to make it look like a textbox. In Firefox, for example, when you click it, an ugly border appears around it. But this is something like what you want.
<div contenteditable="true" style="width:200; border:1px solid black; padding:1.5;">
This is your <b style="color:red";>welcome</b> text
</div>
Try using CSS to style it instead of using an inline style. Since you have lots of textboxes it would be better that way. I just did it using an inline style to give you an idea of how you can make it look more like a textbox. Border and a slight padding. And then you'll also need to make it some sort of scrollable div. But that's a different question.
Hope that helps,
Dalal