0.70: What is HTML?

Hyper Text Markup Language (HTML) is a document markup language that defines a set of elements (generally called tags) that describe the layout of content for presentation by Web browsers. As a subset of the Standard Generalized Markup Language (SGML), HTML shares many of SGML's qualities including:

HTML has evolved from just describing the visual appearance of a document (in HTML versions 2.0, 3.0), to providing tags that are recognized by speech synthesizer routines (in HTML version 3.2), to defining a client side scripting platform (in HTML 4.0, DHTML, XHTML).

The following table describes the functions of HTML 4.0, Dynamic HTML (DHTML), and Extensible Hypertext Markup Language (XHTML).

HTML version Function
HTML 4.0 Provides official support for:
  • tables
  • forms
  • style sheets
  • multimedia
  • scripting
  • internationalization or expanded character encoding
DHTML HTML used in combination with the following technologies:
  • cascading style sheets
  • document object model
  • scripting languages
The key element in DHTML is the document object model or DOM, which represents the document in the form of a tree structure with one parent and multiple children.
See article What is the Document Object Model (DOM) for more information.
XHTML A combination of HTML and XML, XHTML is an application of XML.
See article What is XML for more information on XML. In supported browsers, the browser Document Type Definition (DTD) is a combination of XML and HTML 4.0 commands. To use XHTML, you must include the following declaration in your XHTML page:
   <!DOCTYPE html PUBLIC "..//W3C/DTD XHTML 1.0 Strict/EN"  DTD/xhtml-strict.dtd">

The tags in an HTML document generally consist of a starting tag and an ending tag, as for example:

 <strong> some words </strong>  
You can specify tags in lower case, upper case, or in mixed case. This is true for all versions of HTML, but not for XHTML. In XHTML, tags must be lower case.

Some tags require attributes, as for example:

 <font color="#0066FF">
Since attributes require a value, they are suffixed with an equal sign. In the above example:

With the exception of the <HTML> and </HTML> tags, all tags are optional. Again, this is not true for XHTML. In addition to the <HTML> and </HTML> tags, XHTML also requires the head and body tags.