Basic CSS

Loi Ngoc Nguyen, Duy-Ky Nguyen, PhD

What is CSS?

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) : highest priority

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).

CSS Syntax

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 wish 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
}

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>

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>

Remark Do NOT start a class name with a number! It will not work in Mozilla/Firefox.

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}

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
}

Remark Do NOT start an ID name with a number! It will not work in Mozilla/Firefox.

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")}

Remark Do NOT leave spaces between the property value and the units! If you use "margin-left: 20 px" instead of "margin-left: 20px" it will only work properly in IE6 but it will not work in Mozilla/Firefox or Netscape.

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">
<!--
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>

Box Model



{border:width style color;}

Border Order : Top(T) Right(R) Bottom(B) Left(L)

The shortcut order follows clockwise from 12 (T R B L) for all border properties, eg. width, style, color, margin, ...

There's a rule for this shortcut


    4 values                 --> {border:T R B L;}

                       |-----------------------|
    3 values {border:T R B;} --> {border:T R B R;}

                     |-----------------------|
                     | |-----------------------|
    2 values {border:T R;}   --> {border:T R T R;}

    1 values {border:T;}     --> {border:T T T T;}

    4 values                 --> {border:0 0 B L;}

Style Settings

<div style="color:#0000FF">
  <h3>This is a heading</h3>
  <p>This is a paragraph.</p>
</div>

<p>My mother has <span style="color:blue">blue</span> eyes.</p>

Example 1
Execute upon completion of input
<html>
<head>
<style type="text/css">
	body {background-color: yellow}
	h1 {font-size: 36pt}
	h2 {color: blue}
	p {margin-left: 50px}
</style>
</head>
<body>
<h1>This header is 36 pt</h1>
<h2>This header is blue</h2>
<p>This paragraph has a left margin of 50 pixels</p>
</body>
</html>

and here's the output


Example 2
How to place the image on the page. 
<html>
<head>
</head>
<body>

<style type="text/css">
	body {background-color: tan}
	h1 {color:maroon; font-size:20pt}
	hr {color:navy}
	p {font-size:11pt; margin-left: 15px}
	a:link {color:green}
	a:visited {color:yellow}
	a:hover {color:black}
	a:active {color:blue}
</style>


<h1>This is a header 1</h1> <hr />
<p>You can see that the style sheet formats the text</p> <p>
<a href="http://www.w3schools.com" target="_blank">
This is a link</a></p>


</body>
</html>

and here's the output


Example 3
Set an image as the background
<html>
<head>

<style type="text/css">
body 
{
   background-image: url('bgdesert.jpg')
}
</style>

</head>

<body>
</body>

</html>

and here's the output


Example 4 
How to repeat a background image.
<html>
<head>

<style type="text/css">
body
{ 
   background-image: url('bgdesert.jpg');
   background-repeat: repeat
}
</style>

</head>

<body>
</body>
</html>

and here's the output


Example 5 
How to repeat a background image only vertically. 
<html>
<head>

<style type="text/css">
body
{ 
   background-image: url('bgdesert.jpg');
   background-repeat: repeat-y
}
</style>

</head>
<body>
</body>
</html>

and here's the output


Example 6 
How to repeat a background image only horizontally. 
<html>
<head>

<style type="text/css">
body
{ 
   background-image: url('bgdesert.jpg');
   background-repeat: repeat-x
}
</style>

</head>

<body>
</body>
</html>

and here's the output


Example 7 
How to display a background image only one time
<html>
<head>

<style type="text/css">
body
{ 
background-image: url('bgdesert.jpg');
background-repeat: no-repeat
}
</style>

</head>

<body>
</body>
</html>

and here's the output.


Example 8
How to place the image on the page. 
<html>
<head>
<style type="text/css">
body
{ 
   background-image:url('smiley.gif');
   background-repeat:no-repeat;
   background-attachment:fixed;
   background-position:center; 
}
</style>
</head>

<body>
<p><b>Note:</b> For this to work in Mozilla, the background-attachment property must be set to "fixed".</p>
</body>
</html>

and here's the output.


Example 9 
How to position an image on the page using percent. % 
<html>

<head>
<style type="text/css">
body
{ 
   background-image: url('smiley.gif');
   background-repeat: no-repeat;
   background-attachment:fixed;
   background-position: 30% 20%; 
}
</style>
</head>

<body>
<p><b>Note:</b> For this to work in Mozilla, the background-attachment property must be set to "fixed".</p>
</body>
</html>

and here's the output.



Example 10 
How to position an image on the page using pixels. 
<html>

<head>
<style type="text/css">
body
{ 
   background-image: url('smiley.gif');
   background-repeat: no-repeat;
   background-attachment:fixed;
   background-position: 50px 100px;
}
</style>
</head>

<body>
<p><b>Note:</b> For this to work in Mozilla, the background-attachment property must be set to "fixed".</p>
</body>

</html>

and here's the output.


Example 11 
How to set a fixed background image. The image will not scroll with the rest of the page. 
<html>

<head>
<style type="text/css">
body 
{
background-image: 
url('smiley.gif');
background-repeat: 
no-repeat;
background-attachment: 
fixed
}
</style>
</head>

<body>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
<p>The image will not scroll with the rest of the page</p>
</body>

</html>

and here's the output.



Example 12 
How to use the shorthand property for setting all of the background properties in one declaration. 
<html>

<head>
<style type="text/css">
body
{ 
background: #00ff00 url('smiley.gif') no-repeat fixed center; 
}
</style>
</head>

<body>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
<p>This is some text</p>
</body>

</html>

and here's the output.


CSS Positioning

Example 13 
How to position an element relative to its normal position.
<html>

<head>
<style type="text/css">
h2.pos_left
{
  position:relative;
  left:-20px
}
h2.pos_right
{
  position:relative;
  left:20px
}
</style>
</head>

<body>
<h2>This is a heading in normal position</h2>
<h2 class="pos_left">This heading is moved left to its normal position</h2>
<h2 class="pos_right">This heading is moved right to its normal position</h2>
<p>Relative positioning moves an element RELATIVE to its original position.</p>
<p>The style "left:-20px" subtracts 20 pixels from the element's original left position.</p>
<p>The style "left:20px" adds 20 pixels to the element's original left position.</p>
</body>
</html>

and here's the output.



Example 14 
How to position an element using an absolute value. 
<html>

<head>
<style type="text/css">
h2.pos_abs
{
position:absolute;
left:100px;
top:150px
}
</style>
</head>

<body>
<h2 class="pos_abs">This is a heading with an absolute position</h2>
<p>With absolute positioning, an element can be placed anywhere on a page. The heading below is placed 100px from the left of the page and 150px from the top of the page.</p>
</body>
</html>

and here's the output.



Example 15 
How to position an element with relative to the browser window. 
<html>

<head>
<style type="text/css">
p.one
{
   position:fixed;
   left:5px;
   top:5px;
}
p.two
{
   position:fixed;
   top:30px;
   right:5px;
}
</style>
</head>
<body>

<p class="one">Some text</p>
<p class="two">Some more text</p>

</body>
</html>

and here's the output.


Example 16 
How to set the top edge of an element using a pixel value. 
<html>

<head>
<style type="text/css">
img
{
   position:absolute;
   top:0px
}
</style>
</head>
<body>

<h1>This is a Heading</h1>
<img src="logocss.gif" width="95" height="84" />
<p>Some text.</p>

</body>
</html>

and here's the output.


Example 17 
How to set the top edge of an element using a percent value. 
<html>

<head>
<style type="text/css">
img
{
   position:absolute;
   top:5%
}
</style>
</head>
<body>

<h1>This is a Heading</h1>
<img src="logocss.gif" width="95" height="84" />
<p>Some text.</p>

</body>
</html>

and here's the output.