It doesn't take original page source. It takes browser generated DOM compliant view source. It omits Doctype declaration and HTML tags.
After installation, to use this, one has to
1. Right click on a page & select 'Start DIFF' (this action saves the current page to memory),
2. Go to the page which one wants to compare (with the saved one) and
3. Right click on the page & select 'Show DIFF'.
This application compares 2 set of texts, line by line.
If someone develops a web application in one's local server, deploys it to external server and want to see the difference between the pages (in both servers), this will be useful then.