Why do different browsers render the same HTML differently?

Question

This is an html page :

Answer 1

You miss the tag , in the first line of page.

Ex:

If you don't put it, IE will activate quirks mode, and very strange things will happen. When a browser activate quirks mode, many things change, as box model.

NOTE: many browsers has quirks mode, not only IE (I know that firefox has too).

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More about quirks mode.

List of doctypes.