What You Should Already Know Before You Learn CSS
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Before you continue with css you should have a basic understanding of the following:
- HTML / XHTML
What is CSS?
- CSS stands for Cascading Style Sheets
- Styles define how to display HTML elements
- Styles were added to HTML 4.0 to solve a problem
- External Style Sheets can save a lot of work
- External Style Sheets are stored in CSS files
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Styles Solved a Big Problem
HTML was never intended to contain tags for formatting a document.
HTML was intended to define the content of a document, like:
<h1>This is a heading</h1>
<p>This is a paragraph.</p>
When tags like <font>, and color attributes were added to the HTML 3.2 specification, it started a nightmare for web developers. Development of large web sites, where fonts and color information were added to every single page, became a long and expensive process.
To solve this problem, the World Wide Web Consortium (W3C) created CSS.
In HTML 4.0, all formatting could be removed from the HTML document, and stored in a separate CSS file.
All browsers support CSS today.
CSS Saves a Lot of Work!
CSS defines HOW HTML elements are to be displayed.
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:
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.
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CSS Example
CSS declarations 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:
Example
|
CSS Comments
Comments are used to explain your code, and may help you when you edit the source code at a later date. Comments are 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; } |
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":
Example
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Do NOT start an ID name with a number! It will not work in Mozilla/Firefox.
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
|
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:
Example
|
Do NOT start a class name with a number! This is only supported in Internet Explorer.
When a browser reads a style sheet, it will format the document according to it.
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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.
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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!
