Cascading Style Sheets - Part 01

Cascading Style Sheets Hatem Mahmoud [email_address]

Part 1

Introduction

What is CSS?

Every web page is composed of HTML (HyperText Mark-up Language) code that describes the content
Example:

<p> An <strong> important </strong><font color="#FFFF00"> paragraph </font> . </p>

Displays:

An important paragraph .

Repetitive and hard to maintain

What is CSS? Layers of a web page:

Content: Text, images, animation, video, etc.
Presentation: How the content will appear to a human through a web browser, text reader, etc.
Behavior: Real-time user interaction with the page: validation, sorting, drag-n-drop, etc.

What is CSS?

CSS separates the presentation from the content

Versions

1996 - CSS1 became a W3C recommendation: fonts, alignment, margins, borders, backgrounds, floating, etc.
1998 – CSS2 with advanced features: table cell display, sheets could import others, targeted different output media, etc.
Some parts of CSS2 were very difficult to implement, so the W3C decided to revise the specification

Versions

The name of the revised version was “Cascading Style Sheets, Level 2 Revision 1”
References to CSS2 usually mean CSS2.1
CSS2.1 is the latest and current revision of the CSS2 specification
CSS3 specification is still in draft but some parts have been implemented by some browsers

Linking CSS to HTML 1) Inline Styles: Using the style attribute which is supported by every HTML tag: <h1 style="color:red;"> My Headline </h1>

The simplest way but repetitive across multiple HTML elements

Linking CSS to HTML 2) Embedded Styles: Using the style tag: <style type="text/css"> h1{ font-family:Verdana } //all h1 tags .warning{ color:red } //tags with this class #footer{ font-size:10px } //tag with this id </style> <h1> My Header </h1> <p class=”warning” > WARNING </p> <div id=”footer” > eSpace 2008 </div>

Repetitive across multiple pages

Linking CSS to HTML 2) External Styles: Using separate files: mypage.html <head> <link type= "text/css" href= "nice.css" /> </head> nice.css h1{ font-family:Verdana } .warning{ color:red } #footer{ font-size:10px }

Linking CSS to HTML

Now, multiple HTML pages can share the same CSS file.

Why CSS?

Flexible, can be applied in several ways
Easy to maintain
Accessibility to different users with different devices.
CSS caching = less bandwidth + fast loading

CSS Syntax

General Syntax

Style definition:

body{font-family:Verdana; font-size:9pt;}

General Syntax

Case insensitive
Whitespace and line breaks have no semantic value
Comments:

/* This is a comment */

Properties

Font:

div { color: black; } span { color: #00003D; font-size: 24px; font-family: Verdana, Arial; font-style: italic; font-weight: bold; text-decoration: underline; text-align:justify; ... }

Properties

Size units ( relative and absolute ):

px = Pixels on the screen em = Current font size ex = Height of lowercase "x" mm = Millimeters cm = Centimeters in = Inches (1 inch = 2,54 centimeters) pt = Points (1 point = 1/72 inches) pc = Picas (1 pica = 12 points)

Properties

Backgrounds:

div { background-color: Black; } body { background-image: url(logo.gif); background-color: white; background-attachment: fixed; background-position: right top; background-repeat: no-repeat; } body { background: white url(logo.gif) repeat-x fixed right top; }

Selectors 1) Universal selector: * { margin: 0; padding: 0; } 2) Element type selector: span { font-family: Verdana } 3) Class selector: p.big { font-weight: bold; }

Selectors 4) ID selector: #menu { font-size: 22pt; } // unique id 5) Attribute selector: input[type="submit"] { color: blue; }

Selectors

CSS3 new attribute selectors:

a[href ^ ="http:"] { ... } /* matches a elements whose href attribute value starts with "http:" */ img[src $ =".png"] { ... } /* matches img elements whose src attribute value ends with ".png" */ div[id * ="foo"] { ... } /* matches div elements whose id attribute value contains "foo" */

Selectors

Grouping:

div, p { font-family: Verdana } a img { border: none } ul li ol li { color: blue } #menu a, div li, .note { color: red }

Selectors

Child selector:

ul>li { ... } <ul> <li> <ol> <li> Will not be matched. </li> </ol> </li> </ul>

Selectors

Adjacent sibling selector:

- sibling = has the same parent element - adjacent = immediately following h2+p { ... } <div> <h2> Heading </h2> <p> Will be matched. </p> <p> Will not be matched. </p> </div>

Selectors

General sibling selector:

- sibling = has the same parent element - general = just following h2~p { ... }

Selectors

Example: h2~p { ... }

<p> Will not be matched. </p> <h2> Heading </h2> <p> Will be matched. </p> <div> <p> Will not be matched. </p> </div> <p> Will be matched. </p>

Pseudo-classes (implicit) a:link { ... } //Normal a:visited { ... } //Visited a:hover { ... } //Mouse hovers a.menu:hover { ... } a:active { ... } // Clicking textarea:focus { ... } li:first-child { ... } :lang(fr) { ... }

Pseudo-classes (implicit)

New in CSS3

:nth-child(N) :nth-last-child(N) :nth-of-type(N) :nth-last-of-type(N) :last-child :first-of-type :last-of-type :only-child :only-of-type :root :empty :target :enabled :disabled :checked :not(S)

Pseudo-elements (virtual)

In CSS1

p :first-letter { ... } p :first-line { ... }

New in CSS2

#breadcrumbs :before { content : "You are here:"; margin-right: 0.5em; } span.centimeters :after { content : "cm"; color: #cccccc; }

New in CSS3

:: selection { ... } //represents a part of the document that’s been highlighted by the user, including text in editable text fields

The Cascade

Style declarations cascade down to elements from many origins.
The cascade combines the importance, origin, specificity, and source order of the style declarations to determine which declaration should be applied to a given element.

The Cascade

Importance:

//Normal declaration p {font-size: 1em} //Important declaration p {font-size: 1em !important ;}

The Cascade

Importance and origins priorities (low to high):

1. User agent declarations 2. Normal declarations in user style sheets 3. Normal declarations in author style sheets 4. Important declarations in author style sheets 5. Important declarations in user style sheets

The Cascade

Specificity:

When multiple declarations (with the same importance and origin) try to set the same property to an element, the declaration with the most specific selector will take precedence.

The Cascade

Calculating specificity:

1. Inline styles (highest specificity) 2. Count ID selectors 3. Count class selectors ( .test ), attribute selectors ( [type="submit"] ), and pseudo-classes ( :hover ) 4. Count element type selectors ( div ) and pseudo-elements ( :first-letter )

The Cascade

The process of the cascade:

1. For a given property, find all declarations that apply to a specific element. (user agent, author, user-defined). 2. Sort according to levels of importance and origins. 3. Sort declarations with the same level of importance and origin by selector specificity. 4. If declarations have the same level of importance, origin, and specificity, sort them by the order in which they’re specified

Inheritance

The process by which properties are passed from parent to child elements without explicit definition

div { font-size: 20px; } <div> <p> My <em> cool </em> paragraph is <a href="#"> here </a>. </p> </div>

Inheritance

Exceptions ( borders, backgrounds, etc.)
But you can enforce it:

p { background-image: inherit ; }

CSS Layout

Block vs Inline 1) HTML block-level elements:

May contain inline elements and other blocks
Begin on new lines
Examples: <h1>..<h6> , <p> , <ul> , <ol> , <li> , <table> , <tr> , <th> , <td> , <form> , <select> , <input> , <div> , etc.

Block vs Inline 2) HTML Inline (text-level) elements:

Must be nested within blocks
May contain only text and other inline elements
Don't begin on new lines
Examples: <em> , <strong> , <a> , <img> , <abbr> , <span> , etc.

Block vs Inline

Using the display property:

#menu li { display: inline ; } #menu a { display: block ; }

CSS does not affect the markup: Setting the display property to block for a span element doesn’t allow you to nest an h1 element inside it, because the HTML document type definition forbids that.

Browser Work 1) Parsing: The browser reads the markup and builds a document object model (DOM) tree of nodes.

Browser Work

DOM Tree:

Browser Work 2) Rendering:

Each node in the DOM tree is rendered as zero or more boxes
Inline elements generate inline boxes
Block elements generate block boxes

CSS Box

Box Model (for block-level elements):

CSS Box

Internet Explorer box model bug:

CSS Box

Dimensions:

width = 352px
height = 252px

CSS Box

Take care: width=100%
Any margin, padding, or border will damage the layout
Solutions:

1. Leave with the default value: width=auto (doesn't work with a floated element) 2. Use a fixed width: width=500px 3. Apply the margin, padding, or border to a nested element

CSS Box

Borders:

div { border-style: solid; border-width: 1px; border-color: black; } div { border: dashed 1px black; } div { border-left: dotted 1px black; }

CSS Box

Margins and paddings:

div { margin: 10px } div { padding-right: 10px } div { margin: 10px 5px 0 3px } //top right bottom left div { padding: 10px 5px 3px } //top right-left bottom div { margin: 10px 3px } //top-bottom right-left

CSS Box

Collapsing margins :
when the vertical margins of two elements are touching, only the margin of the element with the largest margin value will be honored
If one element has a negative margin, the margin values are added together
If both are negative, the greater negative value is used.

CSS Box

Example :

h1 { margin: 0 0 25px 0; } p { margin: 20px 0 0 0; } <h1> Heading Content </h1> <p> Paragraph content </p>

CSS Box

To avoid collapsing, use a border or padding:

h1 { margin: 0; } div { margin: 40px 0 0 0; border: 1px solid #000; } p { margin: 20px 0 0 0; } <h1> Heading Content </h1> <div> <p> Paragraph content </p> </div>

Positioning

The CSS position property takes the values:

static The default position in the page flow relative Relative to its normal position in the flow absolute Relative to its containing block (the first ancestor that is not static ) fixed Relative to the browser window (regardless of scrolling)

To change the position of a box, use the top, left, bottom, right properties (no effect with static )

Positioning

Boxes are positioned in three dimensions
To change the stack level, use z-index (default value is 0 )

img.bg{ position: absolute ; left:50px; top:0; z-index: 1 }

Floating

A floated element is one whose float property has the value right or left (the default is none )
Floating an element converts it to a block
A floated box is shifted to the left or right as far as possible
A floated box must have an explicit width. Otherwise, the results can be unpredictable

Floating

Elements below a floated element will wrap around it:

Floating

To avoid this, you can apply the clear property to the following element using: clear:left , clear:right or clear:both

p2{ clear: left ; }

Interaction

If display:none , no box is generated, so float and position are ignored
If position:absolute or position:fixed , float is ignored

Workarounds and Hacks

IE Conditional Comments

Conditional comments are Microsoft’s recommended mechanism for delivering targeted CSS to Internet Explorer

IE Conditional Comments

Star Selector Hack

Using the * html selector
Should apply the rule to any html element that’s the descendant of any other element
As html is the root element, it’s never a descendant of any other element
IE 6 and earlier versions don't understand this!

Star Selector Hack