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Cascading Style Sheets Essay, Research Paper

What is CSS?

CSS (cascading stylesheets) is a simple mechanism for

controlling the style of a Web document without compromising

its structure. By separating visual design elements (fonts,

colors, margins, and so on) from the structural logic of a Web

page, CSS give Web designers the control they crave without

sacrificing the integrity of the data – thus maintaining its

usability in multiple environments. In addition, defining

typographic design and page layout from within a single,

distinct block of code – without having to resort to image

maps, tags, tables, and spacer GIFs – allows for faster

downloads, streamlined site maintenance, and instantaneous

global control of design attributes across multiple pages.

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How CSS Works

CSS overrides the browser’s default settings for interpreting

how tags should be displayed, letting you use any HTML

element indicated by an opening and closing tag (including the

tag) to apply style attributes defined either locally or in a

stylesheet.

Stylesheets contain rules, composed of selectors and

declarations that define how styles will be applied. The selector

(a redefined HTML element, class name, or ID name) is the link

between the HTML document and the style. There are two

different kinds of selectors: types (HTML element tags) and

attributes (such as class and ID names).

A CSS declaration has two parts,

a property (”color”) and a value (”red”).

The basic syntax of a rule

selector {property 1: value 1; property 2: value:

2}

An example (containing two declarations, as above)

P {font-size: 8pt; color: red}

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Local, Global, and Linked Stylesheets

Local (inline) stylesheet declarations, specific to a single

instance on a page, can be used instead of tags to

specify font size, color, and typeface and to define margins,

leading, etc.

This

is a local stylesheet declaration.

Global (embedded) stylesheet declarations, applicable to an

entire document, are defined within the and

tags, which precede the tag in the HTML document

and are usually placed in the header.

To embed a global stylesheet in your HTML document:

Title

[DOCUMENT BODY GOES HERE]

Linked stylesheet declarations use a single stylesheet (in a

separate file, saved with the .css suffix) to define multiple

pages. A typical .css file is a text file containing style rules, as

here:

P {font-family: non-serif; font-size: medium;

color: red}

H1 {font-family: serif; font-size: x-large; color:

green}

H2 {font-family: serif; font-size: large; color:

blue}

To apply a .css stylesheet (”style.css” in the example below)

to an HTML page, a tag is added to the page header:

Inheritance

In cases where local, global, and linked style definitions

conflict, the most specific stylesheet will generally take

precedence: local overrides global, global overrides linked.

Similarly, inline style attributes override ID, ID overrides class,

and class overrides stylesheet-defined HTML elements.

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Units of Measure

Throughout this CSS reference guide, abstract values (notably

values involving units of measure) appear between angle

brackets: . They should be replaced by actual values as

indicated in the key below.

Length Units

indicates a number followed by a unit of measure:

24px.

The number can be an integer or a decimal fraction, and can

be preceded by + or -.

Units can be absolute or relative:

Absolute: mm, cm, in, pt, pc (millimeters,

centimeters, inches, points, picas)

Relative: em, ex, px (the element’s font height,

the element’s x-height, pixels)

Font size may be defined in points, pixels, inches,

or centimeters (pt, px, in, cm) or as a percentage.

can be: xx-small, x-small, small,

medium, large, x-large, xx-large.

can be: larger, smaller.

Percentage Units

indicates a number followed by a % sign: 50%.

In the text-indent, margin, padding, and width properties,

percentage values are relative to the width of the parent

element.

In the font-size property, percentage values are relative to the

font size of the parent element.

In values, percentages can be used to express RGB

values.

Color Units

can represent either or values, as

defined below:

can be: aqua, black, blue, fuchsia,

gray, green, lime, maroon, navy, olive, purple, red,

silver, teal, white, yellow

can be: # rgb(,

, )

rgb(,,)

represents a hexidecimal value, a

decimal value, and a percentage.

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CSS-P (Position)

CSS-P allows you to explicitly determine the position of HTML

elements, providing powerful layout control both for static

documents and for dynamic, animated HTML-based content.

There are two basic methods of positioning HTML elements

using the position property. Absolute position

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