The way to get comfortable with moving objects in display space is to do the exercise indicated below: write the minimal HTML indicated and change the position of the div object. Do this many times. Try to predict where the object will be. This also will help you understand the scale of the pixel unit.
The original HTML was intended for sharing information and not for fancy layout. Cascading Style Sheets is a technology invented to improve the formatting of HTML pages. We will use CSS for positioning elements on the display. This is called Dynamic HTML. One problem is that the different browsers implemented CSS in slightly different ways. More formally, the Document Object Model defining parts of the HTML document are not the same in different browsers.
CSS introduces a new tag called div to define for your own use. The div tag does not have any special meaning, like <b> or <table>. You can give it a name or id and you can put things inside the div element, that is, after the <div> tag and before the </div> tag. The div can be referenced in a style definition, to be demonstrated next. For this example, we will use div to define what will be the bouncing ball. The code in the body of the HTML document is