How does sendtokindle/Reader/Reading View works?

Question

As Tile says, I know there is couple browser e.g. Safari, Firefor, Amazon Silk browser supporting the "reading mode" which can turn any blog/web/news article into a pure text article without any other elements(beside the pics already in the article).

I was wondering how the "reading mode" could be implemented through a web based application ?(I knew the "SendToKindle" plugin for Chrome is something have this kind of function) But I have no idea how this is been implemented.

Any one knows about it may point me to the right direction. Highly appreciated.(Some framework, algorithm, tools or whatever it is)

Thank you very much