HTML is great for marking up content for display purposes.
You probably know that HTML stands for Hypertext Markup Language. The key word here is "markup." You mark up the page with tags so that the page is displayed in a certain way. In this illustration, some text has been marked up with <b> tags, so it will be displayed as bold.
The result of all of this is that HTML can make great looking pages. But what about interactive effects—where someone hovers over or clicks something and the page responds in some way? What makes that possible?