What Is CSS?

Cascading Style Sheets (CSS) is a style sheet language used to describe the presentation semantics (that is, the look and formatting) of a document written in a markup language. It’s most common application is to style web pages written in HTML and XHTML, but the language can also be applied to any kind of XML document, including SVG and XUL.

With CSS we can do

• Enable the separation of document content (written in HTML or a similar markup language) from document presentation, including elements such as the layout, colors, and fonts. This separation can improve content accessibility, provide more flexibility and control in the specification of presentation characteristics,
• Enable multiple pages to share formatting, and reduce complexity and repetition in the structural content.
• CSS can also allow the same markup page to be presented in different styles for different rendering methods, such as on-screen, in print, by voice and on Braille-based, tactile devices.
• CSS specifies a priority scheme to determine which style rules apply if more than one rule matches against a particular element. In this so-called cascade, priorities or weights are calculated and assigned to rules, so that the results are predictable.
• Styles are normally saved in external .css files. External style sheets enable you to change the appearance and layout of all the pages in a Web site, just by editing one single file!

CSS Syntax

A CSS rule has two main parts: a selector, and one or more declarations:

selector { property: value; property: value; property: value……. }

The selector is the (X)HTML element that you want to style. The property is the actual property title, and the value is the style you apply to that property.

Each selector can have multiple properties, and each property within that selector can have independent values. The property and value are separated with a colon and contained within curly brackets. Multiple properties are separated by a semi colon. Multiple values within a property are separated by commas, and if an individual value contains more than one word you surround it with quotation marks. As shown below.

body

{
  background: #eeeeee;
  font-family: "Trebuchet MS", Verdana, Arial, serif;
}

As you can see in the above code I have separated the color from the font-family with a semi-colon, separated the various fonts with commas and contained the "Trebuchet MS" within quotations marks. The final result sets the body color to light grey, and sets the font to ones that most users will have installed on there computer.

You can combine elements within one selector in the following fashion.

h1, h2, h3, h4, h5, h6

{
  color: #009900;
  font-family: Georgia, sans-serif;
}

As you can see in the above code, I have grouped all the header elements into one selector. Each one is separated by a comma. The final result of the above code sets all headers to green and to the specified font. If the user does not have the first font I declared it will go to another sans-serif font the user has installed on their computer.

CSS Id and Class

In addition to setting a style for a HTML element, CSS allows you to specify your own selectors called "id" and "class".

The id Selector
The id selector is used to specify a style for a single, unique element.
The id selector uses the id attribute of the HTML element, and is defined with a "#".
The style rule below will be applied to the element with id="para1":
Example

#para1

{
text-align:center;
color:red;
} 

The class Selector
The class selector is used to specify a style for a group of elements. Unlike the id selector, the class selector is most often used on several elements.
This allows you to set a particular style for any HTML elements with the same class.
The class selector uses the HTML class attribute, and is defined with a "."
In the example below, all HTML elements with class="center" will be center-aligned:
Example
.center {text-align:center;} 

Three Ways to Insert CSS

There are three ways of inserting a style sheet:

• External style sheet
• Internal style sheet
• Inline style

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>

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 work in IE, but not 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 of an HTML page, 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>

Inline Styles
An inline style loses many of the advantages of style sheets by mixing content with presentation. Use this method sparingly!
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>

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.

Multiple Styles Will Cascade into One

Styles can be specified:

• inside an HTML element
• inside the head section of an HTML page
• 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:

• Browser default
• External style sheet
• Internal style sheet (in the head section)
• 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 defined inside the <head> tag, or in an external style sheet, or in a browser (a default value).

Note: If the link to the external style sheet is placed after the internal style sheet in HTML <head>, the external style sheet will override the internal style sheet!

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