If you&#x27;ve ever created a user interface in HTML, you&#x27;ve used something that does not have the concept of user controls.  Let&#x27;s say that I want to make a numeric entry user control for entering numbers on a touch screen.  This control will be made up of a collection of built-in UI controls that will work together to do what I need: let&#x27;s say a text field, a couple of up and down buttons, an always-visible keypad, and a little slider that lets us move between min and max.<p>In most HTML-based systems, it&#x27;s not a built-in or natural thing to have such a self-contained &quot;User Control&quot; that I can just plop in to my user interface in 25 different places and have it manage itself, and the interaction between its own sub-components.  Django, for example, completely lacks such a concept (although TurboGears does have it).  This is a first class concept in classic Windows programming, and also in Flux-- although it&#x27;s missing from most web-based systems.