The notions between a page’s source and a page’s DOM are similar, but different. The source is the raw HTML that is unadulterated by any client-side scripts. It is the direct response of the HTTP request to the server. The DOM, on the other hand, is the same HTML structure that has been modified by JavaScript.
Source Code reads the page’s HTML as if you opened it in a text editor. The source code reflects your HTML structure before any JavaScript is loaded. While the contents can’t be edited, it’s useful to see the HTML Safari receives from the server.
https://developer.apple.com/library/mac/documentation/AppleApplications/Conceptual/Safari_Developer_Guide/ResourcesandtheDOM/ResourcesandtheDOM.html
Hope it helps you.
Edit: Try to read about API, I think there are API's for translate and stuff
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