How does form autofill work in modern web browsers? Which are the most common techniques used in browsers that implement automatic form filling?
-- EDIT --
The question is not about autocomplete, is about form autofilling, which cares not only about the previously inputted values but also considers the meaning and structure of the field to be completed. Google Chrome implementation, for example, tries to parse the inputted fields to guess their type and structure. Or at least is that what I understood from the code linked above.
Take a look over at this answer by kmote.
Highlight is that the browser looks at the field's name tag and makes an educated guess at what sort of data would go there (regex matching is a good naive way to do this). Chrome is working to get some sort of standardization so that this isn't quite as hit-or-miss.