Basic HTML data types 

URLs 

The type "url" refers to either an absolute or relative Universal Resource Locator. Please consult the section on URLs for more details.

Character data 

The syntax of valid character data in HTML is defined in terms of the SGML concepts of NAME and CDATA. For more introductory information about SGML, please consult the SGML tutorial. For more information about SGML, please consult the SGML handbook ([GOLD90]).

• NAME. SGML name tokens must begin with a letter (A-Z and a-z) and may be followed by any number of letters, digits, hyphens ("-") and periods (".").
• CDATA. For attribute values, this refers to a sequence of characters from the document character set, which may include character entities. User agents should resolve CDATA before further processing of attribute values. The HTML 4.0 specification often places further constraints on the permitted syntax for CDATA attributes.

  Although the STYLE and SCRIPT elements use CDATA for their data model, for these elements, CDATA must be handled differently by user agents. Markup and entities must be treated as raw text and passed to the application as is. The first occurrence of the character sequence "</" is treated as terminating the end of the element's content. In valid documents, this would be the end tag for the element.

Colors 

The attribute value type "color" refers to color definitions as specified in [SRGB]. A color value may either be a hexadecimal number (prefixed by a hash mark) or one of the following sixteen color names:

Color names and sRGB values

	Black = "#000000"		Green = "#008000"
	Silver = "#C0C0C0"		Lime = "#00FF00"
	Gray = "#808080"		Olive = "#808000"
	White = "#FFFFFF"		Yellow = "#FFFF00"
	Maroon = "#800000"		Navy = "#000080"
	Red = "#FF0000"		Blue = "#0000FF"
	Purple = "#800080"		Teal = "#008080"
	Fuchsia = "#FF00FF"		Aqua = "#00FFFF"

Thus, the color values "#800080" and "Purple" both refer to the color purple.

Notes on using colors 

Although colors can add significant amounts of information to document and make them more readable, please consider the following guidelines when including color in your documents:

• The use of HTML elements and attributes for specifying color is deprecated. You are encouraged to use style sheets instead.
• Don't use color combinations that cause problems for people with color blindness in its various forms.
• If you use a background image or set the background color, then be sure to set the various text colors as well.
• Colors specified with the BODY and FONT elements and bgcolor on tables look different on different platforms (e.g., workstations, Macs, Windows, and LCD panels vs. CRTs), so you shouldn't rely entirely on a specific effect. In the future, support for the [SRGB] color model together with ICC color profiles should mitigate this problem.
• When practical, adopt common conventions to minimize user confusion. Unless, of course, your desired effect is to confuse users!

Lengths and Pixels 

Values of the type "length" may either be specified as an integer representing the number of pixels of the canvas (screen, paper) or as a percentage of the available horizontal or vertical space. The HTML DTD generally uses %Length for length values that permit percentages and %Pixels for values that only permit pixels.

Thus, the value "50" means fifty pixels. For widths, the value "50%" means half of the available horizontal space (between margins, within a table cell, etc.). For heights, the value "50%" means half of the available vertical space (in the current window, the current table cell, etc.).

For normative information about the definition of a pixel, please consult [CSS1].