Css
What is CSS?
Cascading Style Sheets, fondly referred to as CSS, is a simple design language intended to
simplify the process of making web pages presentable.
CSS handles the look and feel part of a web page. Using CSS, you can control the color of the
text, the style of fonts, the spacing between paragraphs, how columns are sized and laid out,
what background images or colors are used, as well as a variety of
other effects.
CSS is easy to learn and understand but it provides a powerful control over the presentation
of an HTML document. Most commonly, CSS is combined with the markup languages HTML
or XHTML.
Advantages of CSS
CSS saves time - You can write CSS once and then reuse the same sheet in multiple
HTML pages. You can define a style for each HTML element and apply it to as many
web pages as you want.
Pages load faster - If you are using CSS, you do not need to write HTML tag
attributes every time. Just write one CSS rule of a tag and apply it to all the
occurrences of that tag. So, less code means faster download times.
Easy maintenance - To make a global change, simply change the style, and all the
elements in all the web pages will be updated automatically.
Superior styles to HTML - CSS has a much wider array of attributes than HTML, so
you can give a far better look to your HTML page in comparison to HTML attributes.
Multiple Device Compatibility - Style sheets allow content to be optimized for more
than one type of device. By using the same HTML document, different versions of a
website can be presented for handheld devices such as PDAs and cellphones or for
printing.
Global web standards – Now HTML attributes are being deprecated and it is being
recommended to use CSS. So it’s a good idea to start using CSS in all the HTML pages
to make them compatible with future browsers.
Who Creates and Maintains CSS?
CSS is created and maintained through a group of people within the W3C called the CSS
Working Group. The CSS Working Group creates documents called specifications. When a
specification has been discussed and officially ratified by the W3C members, it becomes a
recommendation.
These ratified specifications are called recommendations because the W3C has no control over
the actual implementation of the language. Independent companies and organizations create
that software.
NOTE: The World Wide Web Consortium or W3C is a group that makes recommendations
about how the Internet works and how it should evolve.
CSS Versions
Cascading Style Sheets level 1 (CSS1) came out of W3C as a recommendation in December
1996. This version describes the CSS language as well as a simple visual formatting model
for all the HTML tags.
CSS2 became a W3C recommendation in May 1998 and builds on CSS1. This version adds
support for media-specific style sheets e.g. printers and aural devices, downloadable fonts,
element positioning and tables.
A CSS comprises of style rules that are interpreted by the browser and then applied to the
corresponding elements in your document. A style rule is made of three parts:
Selector: A selector is an HTML tag at which a style will be applied. This could be any
tag like <h1> or <table> etc.
Property: A property is a type of attribute of HTML tag. Put simply, all the HTML
attributes are converted into CSS properties. They could be color, border, etc.
Value: Values are assigned to properties. For example, color property can have the
value either red or #F1F1F1 etc.
You can put CSS Style Rule Syntax as follows:
selector { property: value }
Example: You can define a table border as follows:
table{ border :1px solid #C00; }
Here table is a selector and border is a property and the given value 1px solid #C00 is the
value of that property.
You can define selectors in various simple ways based on your comfort. Let me put these
selectors one by one.
The Type Selectors
This is the same selector we have seen above. Again, one more example to give a color to all
level 1 headings:
h1 {
color: #36CFFF;
}
The Universal Selectors
Rather than selecting elements of a specific type, the universal selector quite simply matches
the name of any element type:
* {
color: #000000;
}
This rule renders the content of every element in our document in black.
The Descendant Selectors
Suppose you want to apply a style rule to a particular element only when it lies inside a
particular element. As given in the following example, the style rule will apply to <em>
element only when it lies inside the <ul> tag.
ul em {
color: #000000;
}
The Class Selectors
You can define style rules based on the class attribute of the elements. All the elements having
that class will be formatted according to the defined rule.
.black {
color: #000000;
}
This rule renders the content in black for every element with class attribute set to black in our
document. You can make it a bit more particular. For example:
h1.black {
color: #000000;
}
This rule renders the content in black for only <h1> elements with class attribute set to black.
You can apply more than one class selectors to a given element. Consider the following
example:
<p class="center bold">
This para will be styled by the classes center and bold.
</p>
The ID Selectors
You can define style rules based on the id attribute of the elements. All the elements having
that id will be formatted according to the defined rule.
#black {
color: #000000;
}
This rule renders the content in black for every element with id attribute set to black in our
document. You can make it a bit more particular. For example:
h1#black {
color: #000000;
}
This rule renders the content in black for only <h1> elements with id attribute set to black.
The true power of id selectors is when they are used as the foundation for descendant
selectors. For example:
#black h2 {
color: #000000;
}
In this example, all level 2 headings will be displayed in black color when those headings will
lie within tags having id attribute set to black.
The Child Selectors
You have seen the descendant selectors. There is one more type of selector, which is very
similar to descendants but have different functionality. Consider the following example:
body > p {
color: #000000;
}
This rule will render all the paragraphs in black if they are a direct child of the <body>
element. Other paragraphs put inside other elements like <div> or <td> would not have any
effect of this rule.
The Attribute Selectors
You can also apply styles to HTML elements with particular attributes. The style rule below
will match all the input elements having a type attribute with a value of text:
input[type="text"]{
color: #000000;
}
The advantage to this method is that the <input type="submit" /> element is unaffected, and
the color applied only to the desired text fields.
There are following rules applied to attribute selector.
p[lang] - Selects all paragraph elements with a lang attribute.
p[lang="fr"] - Selects all paragraph elements whose lang attribute has a value of
exactly "fr".
p[lang~="fr"] - Selects all paragraph elements whose lang attribute contains the
word "fr".
p[lang|="en"] - Selects all paragraph elements whose lang attribute contains values
that are exactly "en", or begin with "en-".
Multiple Style Rules
You may need to define multiple style rules for a single element. You can define these rules
to combine multiple properties and corresponding values into a single block as defined in the
following example:
h1 {
color: #36C;
font-weight: normal;
letter-spacing: .4em;
margin-bottom: 1em;
text-transform: lowercase;
}
Here all the property and value pairs are separated by a semicolon (;). You can keep them
in a single line or multiple lines. For better readability, we keep them in separate lines.
For a while, don't bother about the properties mentioned in the above block. These properties
will be explained in the coming chapters and you can find the complete detail about properties
in CSS References.
Grouping Selectors
You can apply a style to many selectors if you like. Just separate the selectors with a comma,
as given in the following example:
h1, h2, h3 {
color: #36C;
font-weight: normal;
letter-spacing: .4em;
margin-bottom: 1em;
text-transform: lowercase;
}
This define style rule will be applicable to h1, h2 and h3 element as well. The order of the list
is irrelevant. All the elements in the selector will have the corresponding declarations applied
to them.
You can combine the various class selectors together as shown below:
#content, #footer, #supplement {
position: absolute;
left: 510px;
width: 200px;
}
3_css inclusion
There are four ways to associate styles with your HTML document. Most commonly used
methods are inline CSS and External CSS.
Embedded CSS -The <style> Element
You can put your CSS rules into an HTML document using the <style> element. This tag is
placed inside the <head>...</head> tags. Rules defined using this syntax will be applied to
all the elements available in the document. Here is the generic syntax:
<head>
<style type="text/css" media="...">
Style Rules
............
</style>
</head>
Attributes
Attributes associated with <style> elements are:
Attribute Value Description
type text/css Specifies the style sheet language as a content-type
(MIME type). This is a required attribute.
media screen
tty
tv
projection
handheld
print
braille
Specifies the device, the document will be displayed
on. Default value is all. This is an optional attribute.
aural
all
Example
Following is an example of embed CSS based on the above syntax:
<head>
<style type="text/css" media="all">
h1{
color: #36C;
}
</style>
</head>
Inline CSS -The style Attribute
You can use style attribute of any HTML element to define style rules. These rules will be
applied to that element only. Here is the generic syntax:
<element style="...style rules....">
Attributes
Attribute Value Description
style style
rules
The value of style attribute is a combination of style declarations
separated by semicolon (;).
Example
Following is the example of inline CSS based on the above syntax:
<h1 style ="color:#36C;"> This is inline CSS </h1>
It will produce the following result:
This is inline CSS
External CSS -The <link> Element
The <link> element can be used to include an external stylesheet file in your HTML document.
An external style sheet is a separate text file with .css extension. You define all the Style
rules within this text file and then you can include this file in any HTML document using <link>
element.
Here is the generic syntax of including external CSS file:
<head>
<link type="text/css" href="..." media="..." />
</head>
Attributes
Attributes associated with <style> elements are:
Attribute Value Description
type text/css Specifies the style sheet language as a content-type (MIME
type). This attribute is required.
href URL Specifies the style sheet file having Style rules. This attribute
is a required.
media screen
tty
tv
projection
handheld
print
braille
Specifies the device the document will be displayed on. Default
value is all. This is an optional attribute.
aural
all
Example
Consider a simple style sheet file with a name mystyle.css having the following rules:
h1, h2, h3 {
color: #36C;
font-weight: normal;
letter-spacing: .4em;
margin-bottom: 1em;
text-transform: lowercase;
}
Now you can include this file mystyle.css in any HTML document as follows:
<head>
<link type="text/css" href="mystyle.css" media="all" />
</head>
Imported CSS -@import Rule
@import is used to import an external stylesheet in a manner similar to the <link> element.
Here is the generic syntax of @import rule.
<head>
<@import "URL";
</head>
Here URL is the URL of the style sheet file having style rules. You can use another syntax as
well:
<head>
<@import url("URL");
</head>
Example
Following is the example showing you how to import a style sheet file into an HTML document:
<head>
@import "mystyle.css";
</head>
CSS Rules Overriding
We have discussed four ways to include style sheet rules in an HTML document. Here is the
rule to override any Style Sheet Rule.
Any inline style sheet takes the highest priority. So, it will override any rule defined in
<style>...</style> tags or the rules defined in any external style sheet file.
Any rule defined in <style>...</style> tags will override the rules defined in any
external style sheet file.
Any rule defined in the external style sheet file takes the lowest priority, and the rules
defined in this file will be applied only when the above two rules are not applicable.
Handling Old Browsers
There are still many old browsers who do not support CSS. So, we should take care while
writing our Embedded CSS in an HTML document. The following snippet shows how to use
comment tags to hide CSS from older browsers:
<style type="text/css">
<!--
body, td {
color: blue;
}
-->
</style>
CSS Comments
Many times, you may need to put additional comments in your style sheet blocks. So, it is
very easy to comment any part in the style sheet. You can simply put your comments inside
/*.....this is a comment in style sheet.....*/.
You can use /* ....*/ to comment multi-line blocks in similar way you do in C and C++
programming languages.
Example
/* This is an external style sheet file */
h1, h2, h3 {
color: #36C;
font-weight: normal;
letter-spacing: .4em;
margin-bottom: 1em;
text-transform: lowercase;
}
/* end of style rules. */
4. CSS ─ MEASUREMENT UNITS
Before we start the actual exercise, we would like to give a brief idea about the CSS
Measurement Units. CSS supports a number of measurements including absolute units such
as inches, centimeters, points, and so on, as well as relative measures such as percentages
and em units. You need these values while specifying various measurements in your Style
rules e.g. border="1px solid red".
We have listed out all the CSS Measurement Units along with proper Examples:
Unit Description Example
%
Defines a measurement as a percentage
relative to another value, typically an
enclosing element.
p {font-size: 16pt; line-height:
125%;}
cm Defines a measurement in centimeters. div {margin-bottom: 2cm;}
em
A relative measurement for the height of
a font in em spaces. Because an em unit
is equivalent to the size of a given font, if
you assign a font to 12pt, each "em" unit
would be 12pt; thus, 2em would be 24pt.
p {letter-spacing: 7em;}
ex
This value defines a measurement
relative to a font's x-height. The x-height
is determined by the height of the font's
lowercase letter x.
p {font-size: 24pt; line-height:
3ex;}
in Defines a measurement in inches. p {word-spacing: .15in;}
mm Defines a measurement in millimeters. p {word-spacing: 15mm;}
pc
Defines a measurement in picas. A pica is
equivalent to 12 points; thus, there are 6
picas per inch.
p {font-size: 20pc;}pt Defines a measurement in points. A point
is defined as 1/72nd of an inch. body {font-size: 18pt;}
px Defines a measurement in screen pixels. p {padding: 25px;}
5. CSS ─ COLORS
CSS uses color values to specify a color. Typically, these are used to set a color either for the
foreground of an element (i.e., its text) or for the background of the element. They can also
be used to affect the color of borders and other decorative effects.
You can specify your color values in various formats. Following table lists all the possible
formats:
Format Syntax Example
Hex Code #RRGGBB p{color:#FF0000;}
Short Hex Code #RGB p{color:#6A7;}
RGB % rgb(rrr%,ggg%,bbb%) p{color:rgb(50%,50%,50%);}
RGB Absolute rgb(rrr,ggg,bbb) p{color:rgb(0,0,255);}
keyword aqua, black, etc. p{color:teal;}
These formats are explained in more detail in the following sections:
CSS Colors -Hex Codes
A hexadecimal is a 6 digit representation of a color. The first two digits (RR) represent a red
value, the next two are a green value (GG), and the last are the blue value (BB).
A hexadecimal value can be taken from any graphics software like Adobe Photoshop, Jasc
Paintshop Pro, or even using Advanced Paint Brush.
Each hexadecimal code will be preceded by a pound or hash sign ‘#’. Following are the
examples to use Hexadecimal notation.
Color Color HEX
