HTML – the language underlying the Web. Hypertext Mark-up Language (better known as HTML) is the standard language used by web page developers to create and format web pages. Special instructions or ‘tags’ are added to the information elements (ie, the text, graphics etc.) on a web page which web browsers can then translate into a presentation that makes visual sense to the web user.
To see the source of this page, right-click your button and choose ‘view source’ (or similar). You will see the HTML coding that underlies this page.