How do you make an HTML document writable in the browser?

Answer 1

Firebug Extension for Firefox pretty much allows you to edit anything.

Answer 2

In firefox > 3 you can use contentEditable for html elements I understood. A tag looks like this:

<div id="myEditableDiv" contentEditable="true">click here to edit ...</div>

you can control the editing process with 'execCommand' (see: msdn for command identifiers)