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
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 World Wide Web Consortium (W3C) - 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.
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.
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).
Advantages of CSS
- CSS
saves time
When most of us first learn HTML, we get taught to set the font face, size, colour, style etc every time it occurs on a page. This means we find ourselves typing (or copying & pasting) the same thing over and over again. With CSS, you only have to specify these details once for any element. CSS will automatically apply the specified styles whenever that element occurs. - Pages
load faster
Less code means faster download times. - Easy
maintenance
To change the style of an element, you only have to make an edit in one place. - Superior
styles to HTML
CSS has a much wider array of attributes than HTML.
Disadvantages of CSS
- Browser
compatibility
Browsers have varying levels of compliance with Style Sheets. This means that some Style Sheet features are supported and some aren't. To confuse things more, some browser manufacturers decide to come up with their own proprietary tags.
Fortunately, browser compatibility is becoming
less of an issue as the latest browser versions are much more
standards-compliant than their earlier counterpa
//priority
four has the highest priority:
Browser default
External style sheet
Internal style sheet (inside the <head> tag)
Inline style (inside an HTML element)
//syntax
The CSS syntax is made up of three parts: a selector, a
property and a value:
selector
{property: value}
body
{color: black}
p
{text-align:center;color:red}
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
}
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>
.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>
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> |
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> |
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")}
|
///Image in
background
<html>
<head>
<style type="text/css">
body
{
background-image:
url('bgdesert.jpg')
}
</style>
</head>
<body>
ji
</body>
</html>
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