If you have ever designed a web page then you will have encountered HTML. Even if you haven't, you will have almost certainly heard about it. HTML stands for Hypertext Markup Language. It is derived from SGML, which was created so that documents could be structured and defined in a standard way.

It's important to see how HTML operates, as it contains everything we have talked about so far - plus some other bits too.

Learning Check: What are elements? What are attributes?

Computers need a standard way of encoding data so that information can be effectively interchanged. The agreement of common terms facilitates that communication, hence the impact of HTTP and HTML on the growth of the web.

ISO 8879 describes SGML, which was defined to allow data and information to be represented, independently of devices or computing platforms. HTML, a language that conforms to SGML, is referred to as a SGML application. The SGML standard is extremely comprehensive and considered too complicated for 'general' internet use.

HTML is a simplified version of SGML, designed to allow content to be presented on the web, in a relatively simple fashion. However HTML cannot be extended beyond presentation, so it is limited to producing human readable documents. Unlike HTML, XML is not concerned with presentation. It is a standard that permits the creation of markup languages. If we can define our own elements, attributes and data types, then we can define the structure of a document - XML allows us to do this, and therefore create machine readable documents.