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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 HTML Help.
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:
- Browser default
- External style sheet
- Internal style sheet (inside the <head> tag)
- 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).
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
}
|
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
}
|
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. |
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}
|
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. |
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
}
|
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! 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> |
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.
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.
W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
| Property | Description | Values | IE | F | N | W3C |
|---|---|---|---|---|---|---|
| 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 |
The CSS text properties allow you to control the appearance of text. It is possible to change the color of a text, increase or decrease the space between characters in a text, align a text, decorate a text, indent the first line in a text, and more.
Browser support: IE: Internet Explorer, F: Firefox, N: Netscape.
W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
| Property | Description | Values | IE | F | N | W3C |
|---|---|---|---|---|---|---|
| color | Sets the color of a text | color | 3 | 1 | 4 | 1 |
| direction | Sets the text direction | ltr rtl |
6 | 1 | 6 | 2 |
| line-height | Sets the distance between lines | normal number length % |
4 | 1 | 4 | 1 |
| letter-spacing | Increase or decrease the space between characters | normal length |
4 | 1 | 6 | 1 |
| text-align | Aligns the text in an element | left right center justify |
4 | 1 | 4 | 1 |
| text-decoration | Adds decoration to text | none underline overline line-through blink |
4 | 1 | 4 | 1 |
| text-indent | Indents the first line of text in an element | length % |
4 | 1 | 4 | 1 |
| text-shadow | none color length |
|||||
| text-transform | Controls the letters in an element | none capitalize uppercase lowercase |
4 | 1 | 4 | 1 |
| unicode-bidi | normal embed bidi-override |
5 | 2 | |||
| white-space | Sets how white space inside an element is handled | normal pre nowrap |
5 | 1 | 4 | 1 |
| word-spacing | Increase or decrease the space between words | normal length |
6 | 1 | 6 | 1 |
The CSS font properties allow you to change the font family, boldness, size, and the style of a text.
Note: In CSS1 fonts are identified by a font name. If a browser does not support the specified font, it will use a default font.
Browser support: IE: Internet Explorer, F: Firefox, N: Netscape.
W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
| Property | Description | Values | IE | F | N | W3C |
|---|---|---|---|---|---|---|
| font |
A shorthand property for setting all of the properties for a font in one declaration | font-style font-variant font-weight font-size/line-height font-family caption icon menu message-box small-caption status-bar |
4 | 1 | 4 | 1 |
| font-family |
A prioritized list of font family names and/or generic family names for an element | family-name generic-family |
3 | 1 | 4 | 1 |
| font-size |
Sets the size of a font | xx-small x-small small medium large x-large xx-large smaller larger length % |
3 | 1 | 4 | 1 |
| font-size-adjust | Specifies an aspect value for an element that will preserve the x-height of the first-choice font | none number |
- | - | - | 2 |
| font-stretch | Condenses or expands the current font-family | normal wider narrower ultra-condensed extra-condensed condensed semi-condensed semi-expanded expanded extra-expanded ultra-expanded |
- | - | - | 2 |
| font-style |
Sets the style of the font | normal italic oblique |
4 | 1 | 4 | 1 |
| font-variant |
Displays text in a small-caps font or a normal font | normal small-caps |
4 | 1 | 6 | 1 |
| font-weight |
Sets the weight of a font | normal bold bolder lighter 100 200 300 400 500 600 700 800 900 |
4 | 1 | 4 | 1 |
The CSS border properties allow you to specify the style and color of an element's border. In HTML we use tables to create borders around a text, but with the CSS border properties we can create borders with nice effects, and it can be applied to any element.
Browser support: IE: Internet Explorer, F: Firefox, N: Netscape.
W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
| Property | Description | Values | IE | F | N | W3C |
|---|---|---|---|---|---|---|
| border | A shorthand property for setting all of the properties for the four borders in one declaration | border-width border-style border-color |
4 | 1 | 4 | 1 |
| border-bottom | A shorthand property for setting all of the properties for the bottom border in one declaration | border-bottom-width border-style border-color |
4 | 1 | 6 | 1 |
| border-bottom-color | Sets the color of the bottom border | border-color | 4 | 1 | 6 | 2 |
| border-bottom-style | Sets the style of the bottom border | border-style | 4 | 1 | 6 | 2 |
| border-bottom-width | Sets the width of the bottom border | thin medium thick length |
4 | 1 | 4 | 1 |
| border-color | Sets the color of the four borders, can have from one to four colors | color | 4 | 1 | 6 | 1 |
| border-left | A shorthand property for setting all of the properties for the left border in one declaration | border-left-width border-style border-color |
4 | 1 | 6 | 1 |
| border-left-color | Sets the color of the left border | border-color | 4 | 1 | 6 | 2 |
| border-left-style | Sets the style of the left border | border-style | 4 | 1 | 6 | 2 |
| border-left-width | Sets the width of the left border | thin medium thick length |
4 | 1 | 4 | 1 |
| border-right | A shorthand property for setting all of the properties for the right border in one declaration | border-right-width border-style border-color |
4 | 1 | 6 | 1 |
| border-right-color | Sets the color of the right border | border-color | 4 | 1 | 6 | 2 |
| border-right-style | Sets the style of the right border | border-style | 4 | 1 | 6 | 2 |
| border-right-width | Sets the width of the right border | thin medium thick length |
4 | 1 | 4 | 1 |
| border-style | Sets the style of the four borders, can have from one to four styles | none hidden dotted dashed solid double groove ridge inset outset |
4 | 1 | 6 | 1 |
| border-top | A shorthand property for setting all of the properties for the top border in one declaration | border-top-width border-style border-color |
4 | 1 | 6 | 1 |
| border-top-color | Sets the color of the top border | border-color | 4 | 1 | 6 | 2 |
| border-top-style | Sets the style of the top border | border-style | 4 | 1 | 6 | 2 |
| border-top-width | Sets the width of the top border | thin medium thick length |
4 | 1 | 4 | 1 |
| border-width | A shorthand property for setting the width of the four borders in one declaration, can have from one to four values | thin medium thick length |
4 | 1 | 4 | 1 |
An outline is a line that is drawn around elements, outside the border edge, to make the element "stand out".
The CSS outline properties sets the outlines around elements. You can specify the style, color, and width of the outline.
Note: Outlines do not take up space, and they do not have to be rectangular.
Browser support: IE: Internet Explorer, F: Firefox, N: Netscape.
W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
| Property | Description | Values | IE | F | N | W3C |
|---|---|---|---|---|---|---|
| outline | A shorthand property for setting all the outline properties in one declaration | outline-color outline-style outline-width |
- | 1.5 | - | 2 |
| outline-color | Sets the color of the outline around an element | color invert |
- | 1.5 | - | 2 |
| outline-style | Sets the style of the outline around an element | none dotted dashed solid double groove ridge inset outset |
- | 1.5 | - | 2 |
| outline-width | Sets the width of the outline around an element | thin medium thick length |
- | 1.5 | - | 2 |
The CSS margin properties define the space around elements. It is possible to use negative values to overlap content. The top, right, bottom, and left margin can be changed independently using separate properties. A shorthand margin property can also be used to change all of the margins at once.
Note: Netscape and IE give the body tag a default margin of 8px. Opera does not! Instead, Opera applies a default padding of 8px, so if one wants to adjust the margin for an entire page and have it display correctly in Opera, the body padding must be set as well!
Browser support: IE: Internet Explorer, F: Firefox, N: Netscape.
W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
| Property | Description | Values | IE | F | N | W3C |
|---|---|---|---|---|---|---|
| margin | A shorthand property for setting the margin properties in one declaration | margin-top margin-right margin-bottom margin-left |
4 | 1 | 4 | 1 |
| Sets the bottom margin of an element | auto length % |
4 | 1 | 4 | 1 | |
| Sets the left margin of an element | auto length % |
3 | 1 | 4 | 1 | |
| Sets the right margin of an element | auto length % |
3 | 1 | 4 | 1 | |
| margin-top | Sets the top margin of an element | auto length % |
3 | 1 | 4 | 1 |
The CSS padding properties define the space between the element border and the element content. Negative values are not allowed. The top, right, bottom, and left padding can be changed independently using separate properties. A shorthand padding property is also created to control multiple sides at once.
Browser support: IE: Internet Explorer, F: Firefox, N: Netscape.
W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
| Property | Description | Values | IE | F | N | W3C |
|---|---|---|---|---|---|---|
| padding | A shorthand property for setting all of the padding properties in one declaration | padding-top padding-right padding-bottom padding-left |
4 | 1 | 4 | 1 |
| Sets the bottom padding of an element | length % |
4 | 1 | 4 | 1 | |
| Sets the left padding of an element | length % |
4 | 1 | 4 | 1 | |
| Sets the right padding of an element | length % |
4 | 1 | 4 | 1 | |
| padding-top | Sets the top padding of an element | length % |
4 | 1 | 4 | 1 |
The CSS list properties allow you to place the list-item marker, change between different list-item markers, or set an image as the list-item marker.
Browser support: IE: Internet Explorer, F: Firefox, N: Netscape.
W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
| Property | Description | Values | IE | F | N | W3C |
|---|---|---|---|---|---|---|
| list-style | A shorthand property for setting all of the properties for a list in one declaration | list-style-type list-style-position list-style-image |
4 | 1 | 6 | 1 |
| list-style-image | Sets an image as the list-item marker | none url |
4 | 1 | 6 | 1 |
| list-style-position | Sets where the list-item marker is placed in the list | inside outside |
4 | 1 | 6 | 1 |
| list-style-type | Sets the type of the list-item marker | none disc circle square decimal decimal-leading-zero lower-roman upper-roman lower-alpha upper-alpha lower-greek lower-latin upper-latin hebrew armenian georgian cjk-ideographic hiragana katakana hiragana-iroha katakana-iroha |
4 | 1 | 4 | 1 |
| marker-offset | auto length |
1 | 7 | 2 |
The CSS table properties allow you to set the layout of a table.
Browser support: IE: Internet Explorer, M: Mac IE only, F: Firefox, N: Netscape.
W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).
| Property | Description | Values | IE | F | N | W3C |
|---|---|---|---|---|---|---|
| border-collapse | Sets whether the table borders are collapsed into a single border or detached as in standard HTML | collapse separate |
5 | 1 | 7 | 2 |
| border-spacing | Sets the distance that separates cell borders (only for the "separated borders" model) | length length | 5M | 1 | 6 | 2 |
| caption-side | Sets the position of the table caption | top bottom left right |
5M | 1 | 6 | 2 |
| empty-cells | Sets whether or not to show empty cells in a table (only for the "separated borders" model) | show hide |
5M | 1 | 6 | 2 |
| table-layout | Sets the algorithm used to display the table cells, rows, and columns | auto fixed |
5 | 1 | 6 | 2 |
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