Values and Units Used in this Book

Throughout this book, you'll need to enter different values to define different properties. These values come in various forms, depending on the need of the property. Some values are straightforwarda number is a numberbut others have special units associated with them.

Values in chevrons (< >) represent one type of value (Table i.1). Words that appear in code font are literal values and should be typed exactly as shown.

Table i.1. Value Types

VALUE TYPE	WHAT IT IS	EXAMPLE
<number>	A number	1, 2, 3
<length>	A measurement of distance	1in or size
<color>	A chromatic expression	red
<percentage>	A proportion	35%
<URL>	The absolute or relative path to a file on the Web	http://www.mySite.net/bob/graphics/image1.gif

Length Values

Length values come in two varieties:

• Relative lengths, which vary depending on the computer being used (Table i.2).

  Table i.2. Relative Length Values^[*]

  UNIT	NAME	WHAT IT IS	EXAMPLE
  em	Em	Relative to width of the letter M for the font	3em
  ex	x-height	Relative to height of the lowercase x of that font	5ex
  px	Pixel	Relative to the monitor's resolution	125px

  
  

  ^[*] Internet Explorer treats pixels as an absolute value rather than a relative one.

• Absolute values, which remain constant regardless of the hardware and software being used (Table i.3).

  Table i.3. Absolute Length Values

  UNIT	NAME	WHAT IT IS	EXAMPLE
  pt	Point	72pt = 1inch	12pt
  pc	Picas	1 pc = 12 pt	3pc
  mm	Millimeters	1mm = .24pc	25mm
  cm	Centimeters	1cm = 10mm	5.1cm
  in	Inches	1in = 2.54cm	8.25ini

  
  

I generally recommend using pixel sizes to describe font sizes for the greatest stability between operating systems and browsers.

Color Values

You can describe color on the screen in a variety of ways (Table i.4), but most of these descriptions are just different ways of telling the computer how much red, green, and blue are in a particular color.

Table i.4. Color Values

FORMAT	WHAT IT IS	EXAMPLE
#RRGGBB	Red, green, and blue hex-code value of a color (00-99,AA-FF)	CC33FF or C3F
rgb(R#,G#,B#)	Red, green, and blue numeric-values of a color (0-255)	rgb(204,51,255)
rgb(R%,G%,B%)	Red, green, and blue percentage values of a color (0-100%)	rgb(81%,18%,100%)
<name>	The name of the color	Purple

Percentages

Many of the properties in this book can have a percentage as their value. The behavior of this percentage value depends on the property being used.

URLs

A Uniform Resource Locator (URL) is the unique address of something on the Web. This resource could be an HTML document, a graphic, a CSS file, a JavaScript file, a sound or video file, a CGI script, or any of a variety of other file types. URLs can be local, simply describing the location of the resource relative to the current document; or global, describing the absolute location of the resource on the Web and beginning with http://.

In addition, throughout the book, I use links in the code examples. I use the number sign (#) as a placeholder in links that can be directed to any URL you want:

<a href="#">...</a>

The number sign is shorthand that links to the top of the current page. Replace these with your own URLs as desired.

Browser-Safe Colors

Certain colors always display properly on any monitor. These colors are called browser-safe colors. You'll find them fairly easy to remember because their values stay consistent. In hexadecimal values, you can use any combination of 00, 33, 66, 99, CC, and FF. In numeric values, use 0, 51, 102, 153, 204, or 255. In percentages, use 0, 20, 40, 60, 80, or 100.