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 lets you set an

element’s position arbitrarily – that is, relative to its parent

container and independent of the document’s flow. Relative

position allows an element to be offset relative to its natural

position in the document’s flow.

Position is specified with the top and/or left properties using a

value (relative or absolute as appropriate).

The visibility property sets the display state of the element,

but doesn’t affect its position in the layout: An element takes

up the same space whether hidden or visible.

Z-index is used to specify the stacking order of the

positionable elements above or below other HTML elements.

The number value may be positive or negative, and must be an

integer. Default z-ordering of elements in a document is

back-to-front in the order of their appearance in the HTML.

The overflow element is used to control the display of an

element’s contents in cases where they exceed its given

dimensions. It applies only to elements with the position

property of type “absolute.”

Dynamic aspects of managing positioned elements, like hiding,

displaying, and movement, are implemented using an external

scripting language, such as JavaScript.

The top and left properties may be expressed as percentages.

For other CSS-P properties listed, percentages do not apply.

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Attributes

Class and ID

Classes let you create grouping schemes among styled HTML

tags by adding the style definition of a particular class to the

style definitions of several different tags. In the stylesheet, a

class name is preceded by a period (.) to identify it as such:

.foo {property 1: value 1; property 2: value 2}

A very simple example:

P {font-family: sans-serif; font-size: 10pt}

H1 {font-family: serif; font-size: 30pt}

H2 {font-family: serif; font-size: 24pt}

.red {color: red}

.green {color: green}

.blue {color: blue}

The tags and classes can then be used in combination:

This is rendered as 30-point red

serif text.

This is rendered as 10-point red

sans-serif text.

Or not:

This is rendered as 10-point sans-serif text in

the default color.

The ID attribute is used for a uniquely defined style within a

stylesheet. In the stylesheet, an ID name is preceded by a

hash mark (#) to identify it as such:

#foo {property 1: value 1; property 2: value 2}

Text rendered in the foo style.

Text-Level Attributes: and

The tag is generally used to apply a style to inline

text:

This text is rendered as

foo-style and this is not.

The tag is generally used to apply a style to a block of

text, which can also include other HTML elements:

The “foo” style will be applied to this text, and

to this text as well.

The style attribute provides a way to define a style for a single

instance of an element:

This text is

rendered as red, 10-point type

The class, ID, and style attributed can be applied within the

and elements. Used with class or ID, the

and tags work like customized HTML tags, letting you

define logical containers and apply a style to their contents.

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Conclusion

In conclusion CSS is a dynamic, innovative approach to creating

web sites. CSS’s freedom and reliability of code allows for pages

to be created by the most novice web programer and also allows

for creativity of the more advanced users. CSS is the wave of the

future in web programing, soon all sites will have a stylesheet controling

the style and form.

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Bibliography

http://www.w3.org/pub/WWW/Style/css/draft

Draft of the Cascading Stylesheets proposal

http://www4.inria.fr/

FORM-baesd CSS stylesheet editor.

http://www.ornl.gov/sgml/wg8/wg8home.htm

The ISO/IEC JTC1/SC18/WG8 Web Service — look on this page for links to information about

DSSSL and DSSSL-Lite (another proposed web-stylesheet

language). Other DSSSL information can be found at:

http://occam.sjf.novell.com:8080/dsssl/dsssl96 — The draft DSSSL standard.

http://occam.sjf.novell.com:8080/docs/howto_search_dsssl.html — Suggestions on how to use the underlying

SGML tag structure of the draft standard for searching,

ftp.netcom.com/pub/no/novlepub/dsssl/dsssl-dt — The draft standard in DynaText format. You need

the files dsssl96.zip, dtwin23.zip, readme.txt and unzip.exe. You cannot use pkunzip.

ftp.netcom.com/pub/no/novlepub/dsssl/src — The SGML source files for the DynaWeb draft.

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