I've noticed this in numerous modern websites (e.g. facebook and google image search) where the images below the fold load only when user scrolls down the page enough to bring them inside the visible viewport region (upon view source, the page shows X number of <img> tags but they are not fetched from the server straight away). What is this technique called, how does it work and in how many browsers does it work. And is there a jQuery plugin that can achieve this behavior with minimum coding.

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Bonus: can someone explain if there is a onScrolledIntoView or similar event for HTML elements. If not, how do these plugins work?