The changes can affect both the content and the presentation of the document. In this way, a document interactively and “dynamically” designed. This means it can respond to user input and change without a new document from the web server to be accessed. For example, additional information can be displayed when the user clicks the mouse on a particular HTML element.
What at first sounds like a subtle distinction, a distinction is crunch time: With HTML can be only distinguished texts and build documents. With CSS, these documents are formatted and mockup. In CSS called declarations are written responsive HTML elements and assign formatting.