Setting a dHTML element’s behavior attribute allows you to customize the element.
Microsoft implemented the behavior attribute of Cascading Style Sheets in a way that enabled object-oriented programming to enter the world of web authoring. By encapsulating dHTML in an external object, the properties and methods of that object can be used. A web page can then use these objects with the behavior attribute. This means, for example, that a web author no longer had to perform an explicit browser detection.