Cascading Style Sheets
CSS, or cascading style sheets, are used to describe the visual style and presentation of a document, most commonly web sites. One of the purposes of CSS is to separate the presentation of a document from the structure and content (although it is possible to embed CSS within the structure as well), and is a W3C recommendation. Separation of style from structure and content increases maintainability for the author(s) of a document as well as accessibility for the audience. CSS is commonly applied to HTML, XHTML and XML documents, though it is possible, albeit rare, to apply it to other types of documents as well.
CSS is one way of describing the "style" -- the colors, fonts, layout, and other "presentation" aspects of a document. A single CSS file can describe a common "style" to be applied to many HTML, XHTML, and XML documents (which describe the content and structure of the elements of each document). Typically a particular element in a XHTML file has a "cascade" of CSS style rules that can be applied to it. The highest priority style rule is applied to each element.
Why would I want to use CSS rather than something else?
CSS is a powerful tool that gives web designers flexibility and modularity in the presentation layer of a web site.
CSS allows you to have every format rule defined for later use (here "format" means how things appear). So if you are writing a large website and you want a consistent appearance for every title, sub-title, how examples of code appear, how paragraphs are aligned, (I could go on, CSS covers a wide range of presentation options) then CSS is the way to go.
Let's say you have a 1200 page website that took you months to complete. Your current boss gets a promotion and another person fills his place. Your new boss says to change the font, the size, the background, the appearance of tables, etc. everywhere on your 1200-page site to comply with some new corporate policy. If you engineered your site appropriately with CSS, you could do this by editing one linked CSS file that has all your appearance (format) rules in one place. Or you could do it the hard way, and hammer the appearance changes on each and every of your 1200 HTML pages. By using CSS, changes can be made fast and easy simply by editing a few rules and lines in the global stylesheet. Before CSS was used, these changes were more difficult, expensive, and very time-consuming.
Some server-side template systems can largely be used for the same purpose. Unlike CSS, however, they most often separate the structure from the content and not the presentation from the structure, making it much more difficult for users who wish to disable or ignore styling to do so.
CSS Syntax
A CSS rule has two main parts: a selector, and one or more declarations:
The selector is normally the HTML element you want to style.
Each declaration consists of a property and a value.
The property is the style attribute you want to change. Each property has a value.
CSS Example
A CSS declaration always ends with a semicolon, and declaration groups are surrounded by curly brackets:
p {color:red;text-align:center;}
To make the CSS more readable, you can put one declaration on each line, like this:
p
{
color:red;
text-align:center;
}
{
color:red;
text-align:center;
}
The id and class Selectors
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":
#para1
{
text-align:center;
color:red;
}
{
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 many 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:
.center {text-align:center;}
You can also specify that only specific HTML elements should be affected by a class.
In the example below, all p elements with class="center" will be center-aligned:
p.center {text-align:center;}
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>
<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>
<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
Tip: 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!References :
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