hi ...
yes ... it is a mix of HTML, CSS and JavaScript ... so that you may achieve screen-effects like popupmenus, image-transitions, etc. and page-manipulations like reordrering of table-rows, updating of dependend screenitems and so on. standalone HTML is static ... and with JavaScript and its dom-scripting methods you may create 'dynamic' interfaces on the clientside ... css is used to seperate the formatting from the content so that we manipulate the document-tree with js to create/adapt/update the content and then set the layout with the help of style-sheets ...
kind regards