HTML is an application of the Standard Generalized Markup Language[15] (SGML). SGML is a system used for defining structured document types and markup languages. HTML applies a subset of SGML to markup the text documents found on the World Wide Web with hypertext links, multiple fonts, interactive forms, and images. This markup is included in text documents using what are known as HTML tags. Start tags are contained within the characters characters: '<' and '>'; end tags are contained with the characters '</' and '>'. The text within these characters is used to define the name of the tag and any optional tag attributes. In order to markup a section of text, a tag has both a start and end tag. An example of text marked up with an HTML tag is shown in Figure 2.1.
The tag shown in Figure 2.1 is named H1. Besides having a name, a tag can also have a set of attributes. Tag attributes are specified after the tag name in key="value" pairs, where key is the name of the attribute and value is value assigned to key. Each tag has its own set of accepted attributes.
HTML includes tags for assigning text attributes such as fonts, bold, italics, and color to regions of text. Hypertext tags are used to link text and images to other HTML documents. Also included are tags which are used to define document structure. All HTML documents are supposed to conform to a structure in which specific tags are used to specify the document flow. For example, every HTML document should contain a HEAD and BODY section. HEAD is for header information such as the document TITLE, while BODY is for the document contents. A complete HTML document using most of the commonly found HTML tags is shown in Figure 2.2.
HTML was originally developed by Tim Berners-Lee at CERN. Since that time many revisions and extensions have been made. The following sections will cover the three major revisions that followed the original HTML release.
