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What You Should Already Know
Before you continue you should have some basic understanding of the following:
• HTML / XHTML
If you want to study this subject first, find the tutorials on our Home page.
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What is CSS?
• CSS stands for Cascading Style Sheets
• Styles define how to display HTML elements
• Styles are normally stored in Style Sheets
• Styles were added to HTML 4.0 to solve a problem
• External Style Sheets can save you a lot of work
• External Style Sheets are stored in CSS files
• Multiple style definitions will cascade into one
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CSS Demo
With CSS, your HTML documents can be displayed using different output styles:
See how it works
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Styles Solve a Common Problem
HTML tags were originally designed to define the content of a document. They were supposed to say "This is a header", "This is a paragraph", "This is a table", by using tags like <h1>, <p>, <table>, and so on. The layout of the document was supposed to be taken care of by the browser, without using any formatting tags.
As the two major browsers - Netscape and Internet Explorer - continued to add new HTML tags and attributes (like the <font> tag and the color attribute) to the original HTML specification, it became more and more difficult to create Web sites where the content of HTML documents was clearly separated from the document's presentation layout.
To solve this problem, - the non profit, standard setting consortium, responsible for standardizing HTML - created STYLES in addition to HTML 4.0.
All major browsers support Cascading Style Sheets.
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Style Sheets Can Save a Lot of Work
Styles sheets define HOW HTML elements are to be displayed, just like the font tag and the color attribute in HTML 3.2. Styles are normally saved in external .css files. External style sheets enable you to change the appearance and layout of all the pages in your Web, just by editing one single CSS document!
CSS is a breakthrough in Web design because it allows developers to control the style and layout of multiple Web pages all at once. As a Web developer you can define a style for each HTML element and apply it to as many Web pages as you want. To make a global change, simply change the style, and all elements in the Web are updated automatically.
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Multiple Styles Will Cascade Into One
Style sheets allow style information to be specified in many ways. Styles can be specified inside a single HTML element, inside the <head> element of an HTML page, or in an external CSS file. Even multiple external style sheets can be referenced inside a single HTML document.
Cascading Order
What style will be used when there is more than one style specified for an HTML element?

Generally speaking we can say that all the styles will "cascade" into a new "virtual" style sheet by the following rules, where number four has the highest priority:
1. Browser default
2. External style sheet
3. Internal style sheet (inside the <head> tag)
4. Inline style (inside an HTML element)
So, an inline style (inside an HTML element) has the highest priority, which means that it will override a style declared inside the <head> tag, in an external style sheet, or in a browser (a default value).
Note: If the external style sheet link is placed below the internal style sheet in HTML <head>, the external style sheet will override the internal style sheet.
Syntax
The CSS syntax is made up of three parts: a selector, a property and a value:
selector {property:value}
The selector is normally the HTML element/tag you wish to define, the property is the attribute you wish to change, and each property can take a value. The property and value are separated by a colon, and surrounded by curly braces:
body {color:black}
Note: If the value is multiple words, put quotes around the value:
p {font-family:"sans serif"}
Note: If you want to specify more than one property, you must separate each property with a semicolon. The example below shows how to define a center aligned paragraph, with a red text color:
p {text-align:center;color:red}
To make the style definitions more readable, you can describe one property on each line, like this:
p
{
text-align:center;
color:black;
font-family:arial
}

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Grouping
You can group selectors. Separate each selector with a comma. In the example below we have grouped all the header elements. All header elements will be displayed in green text color:
h1,h2,h3,h4,h5,h6
{
color:green
}

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The class Selector
With the class selector you can define different styles for the same type of HTML element.
Say that you would like to have two types of paragraphs in your document: one right-aligned paragraph, and one center-aligned paragraph. Here is how you can do it with styles:
p.right {text-align:right}
p.center {text-align:center}
You have to use the class attribute in your HTML document:
<p class="right">This paragraph will be right-aligned.</p>
<p class="center">This paragraph will be center-aligned.</p>
Note: To apply more than one class per given element, the syntax is:
<p class="center bold">This is a paragraph.</p>
The paragraph above will be styled by the class "center" AND the class "bold".
You can also omit the tag name in the selector to define a style that will be used by all HTML elements that have a certain class. In the example below, all HTML elements with class="center" will be center-aligned:
.center {text-align:center}
In the code below both the h1 element and the p element have class="center". This means that both elements will follow the rules in the ".center" selector:
<h1 class="center">This heading will be center-aligned</h1>
<p class="center">This paragraph will also be center-aligned.</p>
Do NOT start a class name with a number! It will not work in Mozilla/Firefox.
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Add Styles to Elements with Particular Attributes
You can also apply styles to HTML elements with particular attributes.
The style rule below will match all input elements that have a type attribute with a value of "text":
input[type="text"] {background-color:blue}

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The id Selector
You can also define styles for HTML elements with the id selector. The id selector is defined as a #.
The style rule below will match the element that has an id attribute with a value of "green":
#green {color:green}
The style rule below will match the p element that has an id with a value of "para1":
p#para1
{
text-align:center;
color:red
}
Do NOT start an ID name with a number! It will not work in Mozilla/Firefox.
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CSS Comments
Comments are used to explain your code, and may help you when you edit the source code at a later date. A comment will be ignored by browsers. A CSS comment begins with "/*", and ends with "*/", like this:
/*This is a comment*/
p
{
text-align:center;
/*This is another comment*/
color:black;
font-family:arial
}

How to Insert a Style Sheet
When a browser reads a style sheet, it will format the document according to it. There are three ways of inserting a style sheet:
External Style Sheet
An external style sheet is ideal when the style is applied to many pages. With an external style sheet, you can change the look of an entire Web site by changing one file. Each page must link to the style sheet using the <link> tag. The <link> tag goes inside the head section:
<head>
<link rel="stylesheet" type="text/css" href="mystyle.css" />
</head>
The browser will read the style definitions from the file mystyle.css, and format the document according to it.
An external style sheet can be written in any text editor. The file should not contain any html tags. Your style sheet should be saved with a .css extension. An example of a style sheet file is shown below:
hr {color:sienna}
p {margin-left:20px}
body {background-image:url("images/back40.gif")}
Do not leave spaces between the property value and the units! "margin-left:20 px" (instead of "margin-left:20px") will only work in IE6, but it will not work in Firefox or Opera.
Internal Style Sheet
An internal style sheet should be used when a single document has a unique style. You define internal styles in the head section by using the <style> tag, like this:
<head>
<style type="text/css">
hr {color:sienna}
p {margin-left:20px}
body {background-image:url("images/back40.gif")}
</style>
</head>
The browser will now read the style definitions, and format the document according to it.
Note: A browser normally ignores unknown tags. This means that an old browser that does not support styles, will ignore the <style> tag, but the content of the <style> tag will be displayed on the page. It is possible to prevent an old browser from displaying the content by hiding it in the HTML comment element:
<head>
<style type="text/css">

</style>
</head>
Inline Styles
An inline style loses many of the advantages of style sheets by mixing content with presentation. Use this method sparingly, such as when a style is to be applied to a single occurrence of an element.
To use inline styles you use the style attribute in the relevant tag. The style attribute can contain any CSS property. The example shows how to change the color and the left margin of a paragraph:
<p style="color:sienna;margin-left:20px">This is a paragraph.</p>

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Multiple Style Sheets
If some properties have been set for the same selector in different style sheets, the values will be inherited from the more specific style sheet.
For example, an external style sheet has these properties for the h3 selector:
h3
{
color:red;
text-align:left;
font-size:8pt
}
And an internal style sheet has these properties for the h3 selector:
h3
{
text-align:right;
font-size:20pt
}
If the page with the internal style sheet also links to the external style sheet the properties for h3 will be:
color:red;
text-align:right;
font-size:20pt
The color is inherited from the external style sheet and the text-alignment and the font-size is replaced by the internal style sheet.
CSS Background Properties
The CSS background properties allow you to control the background color of an element, set an image as the background, repeat a background image vertically or horizontally, and position an image on a page.
Browser support: IE: Internet Explorer, F: Firefox, N: Netscape.
: The number in the "" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
Property Description Values IE F N
background A shorthand property for setting all background properties in one declaration background-color
background-image
background-repeat background-attachment background-position 4 1 6 1
background-attachment Sets whether a background image is fixed or scrolls with the rest of the page scroll
fixed 4 1 6 1
background-color Sets the background color of an element color-rgb
color-hex
color-name
transparent 4 1 4 1
background-image Sets an image as the background url(URL)
none 4 1 4 1
background-position Sets the starting position of a background image top left
top center
top right
center left
center center
center right
bottom left
bottom center
bottom right
x% y%
xpos ypos 4 1 6 1
background-repeat Sets if/how a background image will be repeated repeat
repeat-x
repeat-y
no-repeat 4 1 4 1