finding character index of cursor in HTML document

When using webrenderer (which supports Firefox), it would be helpful to find the character index (of the cursor) when a user clicks on the screen or traverses between search terms.  To be more specific, if the plain text (the text that is actually displayed in the browser) for the HTML is 500 characters in length, the goal is to find the index of the cursor (between 1-500) each time it is moved via a mouse click or prev/next search.
joexAsked:
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