You can make individual HTML elements' content editable or the entire HTML page content editable.
Saving a page which the user has edited is less straightforward, there are no API methods to tell you which (if any) HTML elements a user has edited.
So you'd have to use javascript to grab the entire page content if you want to save it.
Tested and works in a Froyo emulator so the WebView does support these methods - try this url in the stock browser or a WebView: HTML5 Demo: ContentEditable.