In the old times, each browser had its own behavior (up to NS4 - IE5.5.)
Modern browsers are supposed to follow the web standard. However, the "standard behavior" would crash websites designed to be seen in old such old browsers. As a result, browser developers decided to create the "quirks mode", and they render websites exactly as NS4 or IE5.5 used to.
Read more about it at https://developer.mozilla.org/En/Mozilla_Quirks_Mode_Behavior and https://developer.mozilla.org/en/Mozilla%27s_Quirks_Mode