css overview. css quick facts css – cascading style sheets can be used in html, svg, and any xml...

CSS Overview

CSS Quick Facts

• CSS – Cascading Style Sheets

• Can be used in HTML, SVG, and any XML document.

• Is a style sheet language used for presentation

Three ways to get CSS into HTML

From easiest to best practice1. Inline2. Embedded3. External

Inline

Using the style attribute

<p style=“color: red; font-family: arial;”>This is awesome content.

Embedded

Placed inside of the <head> element

<head> <style type=“text/css”> p { color: red; font-family: arial; } h1 { margin: 0em; padding: 0em; } </style><head>

External

index.html

style.css

p { color: red; font-family: arial; }h1 { margin: 0em; padding: 0em; }

Inline CSS is Bad!

Avoid this because we want to1. Separate presentation from structure2. Make maintenance easier3. Promote re-usability4. Make our HTML more accessible

Disadvantages of Embedded CSS

1. They affect only the pages they’re on2. Increase page load times

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Why use external CSS?

• See previous two slides

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External CSS Tip

Don’t use @import for external cssNot as network friendly and will not be cached by browser.

**if you don’t know what @import is, don’t worry**

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CSS Selectors

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How does CSS work?

Three key features1. Selector2. Properties3. Values

p { color: red; font-family: arial; }

Property ValueDeclarationSelector

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Selectors

5 ways to select1. element - body2. class - .page3. id - #navigation4. position in document - .page p, p:first-

child, div > a, li + li

5. State – a:visited, input:focus

Selectors

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A Subtle Distinction

Class selectorMatches an element that has a class value which contains the attribute value.

Id selectorMatches an element that has a class value which is equal

Selectors

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Element Selector

• To select an element, create a selector using the element’s name.

p { color: red}

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Style all h2 elements

<body> <h1>History</h1> <section> <article>

<h2>Ancient</h2> <p>Knights and horses use to be best friends.</p> </article> <article> <h2>Colonial</h2> <p>In 1300, men walked around town holding a gun.</p> </article>

</section></body>

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Class Selector

• To select an element by class value, create a selector with the class value you want to select prepended with a period.

<div class=“content”>Hello blue text!</div>

.content { color: blue; }

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Id Selector

• To select an element by id, create a selector with the id value you want to select prepended with a pound/hash/bang sign.

<div id=“nav”>Hello green text!</div>

#nav { color: green; }

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Pseudo-Class Selector

• Create a pseudo-class selector by appending a colon after an element, class, or id selector followed by the pseudo-class.

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Pseudo-Class Selector

<p> The last P before the note. </p><div class="note"> <p id=“fred”>The selector applies to me, I’m purple!</p> <p>The selector doesn’t apply to me</p></div>

div p:first-child { color: purple; }div p#fred:first-child { color: purple; }

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Specialized SelectorsSelector Selects

* anything

> A child

Two or more selectors separated by white space

A descendant (any connected element that is lower down the tree)

+ An adjacent element (sibling)

:first-child First child element

:nth-child(n) The nth child

p:first-line First formatted line of paragraph

p:first-letter First letter of the first line of a block

Selectors

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30 selectors you should memorize

• http://net.tutsplus.com/tutorials/html-css-techniques/the-30-css-selectors-you-must-memorize/

Selectors

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More Info

• See the CSS spec for thorough details on selectors

http://www.w3.org/TR/CSS2/selector.html

Selectors

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What will be selected?

#nav ul.links a:hover

<body> <div id=“nav”> <ul class=“links”> <li><a href=“#”>Selectors Rule</a></li> </ul> </div></body>

A possible HTML snippet

Selectors

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Practice Selectors

Visit Selectoracle to learn about and practice with CSS selectors.

Selectors

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CSS Specificity/Point System

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#nav { color: red; }

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.warning { color: yellow; }p:first-child { padding: 10em; }

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a { margin-top: 5em; }

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• If the element has inline styling, that automatically1 wins 1,0,0,0 points

• For each ID value, apply 0,1,0,0 points

• For each class value (or pseudo-class or attribute selector), apply 0,0,1,0 points

• For each element reference, apply 0,0,0,1 point

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Points System (Bad Way!!!)

• Is NOT base 10 system

• (1, 1, 1, 1)

-> (1 * (10^3)) + (1 * (10^2)) + (1 * (10^1)) + (1 * (10^0))

--> (1 * (1000)) + (1 * (100)) + (1 * (10)) + (1 * (1))

---> (1,000)) + (100) + (10) + (1)

----> 1,111-----> WRONG!!!

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Points System

• Add the number of selectors for each category and place them in the appropriate column.

• Numbers don’t fill over

• (0, 1, 13, 1)

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How to determine a winner

1. Score your selectors in the order they are written

2. Starting from the top, go down the first column looking for the highest score. If you don’t find a winner or two columns are equal, move to the next column (repeat if necessary).

3. If you find two selectors with equal specificity, the selector listed last wins.

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(0, 1, 1, 10)(0, 0, 13, 5)(0, 0, 7, 3)(0, 0, 2, 1)(0, 0, 1, 0)(0, 1, 2, 2)

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(0, 1, 1, 10)(0, 0, 13, 5)(0, 0, 7, 3)(0, 0, 2, 1)(0, 0, 1, 0)(0, 1, 2, 2) – winner!

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Lets Practice

• div #nav li

• ol.ing li

• ol.ing:hover li

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Point System Reminder!

• 13 elements don’t override a class• 13 classes don’t override an id• 13 ids don’t override an inline style

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Inheritance & Units

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Inheritance

• Some styles are inherited by the child from their parent

1. Font-family2. Text-decoration3. Text related styles

Inheritance

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Inheritance: ones on left will, ones on right will not

Inheritance

Text related Layout related

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Inheritance

You can force a child to inherit properties from its parent

<div style=“border: 1px solid red;”> <p style=“border: inherit”> I like to inherit stuff. </p></div>

Inheritance

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• Absolute: always compute the same value– Points (pt)– Inches (in)

• Relative: computed with respect to something– Pixels (px)– Em– %

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Absolute Units

• Don’t use them unless your media type is print

• DON’T USE THEM for building web pages!

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Relative Units : Pixels

• Pixels are relative because they are based on the pixel size of the display screen.

• There are a lot of different pixel sizes so almost every screen will be different.

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Relative Units : Em

For fonts and containers• 1em is equal to the current font size of the

element in question.

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Relative Units : % Percent

For fonts• 100% is equal to the current font size of the

element in question

For containers• 100% is equal to the size of the parent

container

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CSS Font Size Fiddle

• http://jsfiddle.net/znpKn/

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Box Model

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CSS Moment of Clarity

• Every page element is a box.

• Developers can control the size and position of these boxes.

Box Model

CSE 3345 51Box Model

Box Model

• Every page element can specify margin, border, padding, and content.

content

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Content Area

• Where the text, image, content, etc. goes.

• Specifying width and height for an element only affects the content area not the entire box (Unless in quirks mode).

Box Model

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Brief Aside: Quirks vs Strict Mode

• Quirks mode is what old non standard compliant browsers use.

• Strict Mode is for the W3C Standard.

• If you don’t specify a !DOCTYPE, you’re in quirks mode.

• Don’t be in quirks mode, always use a !DOCTYPE.

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Padding: Quick Facts

• Use it to provide padding between borders and content.

• Padding is apart of the box that sits on top of background color.

• Can not have negative values

Box Model

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Padding: Quick Facts

• Beware of padding set by the browser’s default style. It sometimes causes undesirable visuals.– <h1> in particular

• For non-replaced inline elements, padding acts weird. Horizontal values will work as expected; however, vertical values won’t have any effect on content above or beneath it.

Box Model

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Border: Quick Facts

• Adds a border around your content.– Can specify top, right, left, and/or bottom

• Different border styles.– Solid, dashed, double, groove, etc

• The border is apart of the box that sits on top of the background color.

Box Model

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Margin: Quick Facts

• Used to put “blank space” around an object.– Won’t show bg color.

• Can accept negative values– Can cause an element to overlap another element

Box Model

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Margin: Quick Facts

• Use this for centering an element inside its parent horizontally.– When an element specifies width and sets margin-left and margin-right to auto, the element will center itself inside its parent.

• For non-replaced inline elements, margins act weird. Horizontal values will work as expected; however, vertical values won’t have any effect on content above or beneath it.

Box Model

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Margin and Padding Example

• http://jsfiddle.net/blinkmacalahan/mrKPC/

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Margins Collapsing

• More advanced concept, causes hair pulling!

• Horizontal margins never collapse

Box Model

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CSS Moment of Clarity

Adjoining vertical margins of two or more boxes can collapse.

See W3 for details

Box Model

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Understanding Collapsing Margins

• Two adjacent elements with positive margins will collapse and share the larger specified margin.

• Two adjacent elements with one positive and one negative margin will collapse and share a margin that is the summation of the two.

Box Model

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Understanding Collapsing Margins

• Two adjacent elements with one negative and one negative margin will collapse and share the greater margin

• A parent element with no border or padding, will collapse margins with a child element and share the larger of the two margins.

Box Model

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Vertical Margin Collapsing Example

• http://jsfiddle.net/blinkmacalahan/RHrxL/

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Adding it all up

• The computed dimension of an element is equal to

margin + border + padding + (width | height)

• Block elements’ width grow to fit the parent. See display: block section.

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The auto value for Box Model Properties

• For the box model properties, it can only be used on width, height, and margin properties

• Auto values will fill the remaining space needed to fill the parent’s width for block elements.

• Read Chapter 7 in CSS: Def Guide for extremely good coverage on this topic.

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Display Property

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Display

Essentially there are three choices:

1. block2. inline3. none

Display

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Display

In reality there several values for display; however, we won’t focus on the rest.

Display

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Display: Block

• Elements generate "new lines" both before and after their boxes when in the normal flow

• It has a specified width and height

• By default, an element’s width defaults to 100% of their parent’s width. (Unless a width is supplied)

Display

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Display: Block

• If an element’s height is not explicitly set, it is determined by the size of its content.

Display

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Notable Block Elements

• <div>• <p>• <h1-h6>• <form>• <ul>• <li>

Display

HTML5 Elements• <nav>• <section>• <header>• <footer>

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Adding it all up: Block Elements

• The sum of a horizontal block-level element’s box in the normal flow always equals the width of its parent.– Aka expand naturally to fill the parent’s width

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Display: Inline

Often described as the ‘opposite’ of block elements

Display

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Display: Inline

• Elements do not generate "line breaks" before or after themselves

• Elements begin on the same line– This means that inline elements will always be

next to the preceding element, unless it overflows to next line

• Consumes only enough space for its contentDisplay

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Display: Inline

• Width and Height is always calculated based on its content.

• Any dimension specified will be ignored for non-replaced elements.

• Vertical margin and padding values for non-replaced elements have no effect on content above or beneath them

Display

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Notable Inline Elements

• <span>• <a>• <img>• <input>• <address>

Display

HTML5 Elements• <strong>• <em>• <time>

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Display: None

• Elements do not take up any space

• Elements are said to be removed from the document/flow

• Display: none is different than visibility:none

Display

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Position Property

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Position

• Ability to position elements exactly where you want them to be

• You can specify the top, right, bottom, and left CSS properties to offset an element depending on its position value.

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Position values

• Static• Relative• Absolute• Fixed

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Position : static

• The element’s box is generated and laid out as normal

• This is the default value for all elements

• top, right, bottom, and

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Position : relative

• Changes the element’s box position relative to where it would appear

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Position : relativeHTML<div> <span>this is some text</span> <span>I’ll add more text</span></div>

CSSdiv { width: 500px; height: 500px; }span {background-color: red;}span:first-child { position: static; }

This is some text I’ll add more text

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CSSdiv { width: 500px; height: 500px; }span {background-color: red;}span:first-child { position: relative;}

This is some text I’ll add more text

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CSSdiv { width: 500px; height: 500px; }span {background-color: red;}span:first-child { position: relative;

top: 50%; }

This is some text

I’ll add more text

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CSSdiv { width: 500px; height: 500px; }span {background-color: red;}span:first-child { position: relative;

top: 50%; left: 50%;}

This is some text

I’ll add more text

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Position : relative

• Although a relatively positioned element might be shifted from its normal place, the space it would have occupied is not filled.

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Position : absolute

• The absolutely positioned element is placed with respect to its nearest ancestor that has a position value other than static (aka its containing block).

• To find a non static ancestor, look up the tree from the absolute element for an element that specifies a position property other than static.

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Finding a non static ancestor

<html>

<body>

p { position: absolute; }

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Is the <div> non static?

<html>

<body>

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Is the <body> non static?

<html>

<body>

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Is the <html> non static?

<html>

<body>

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Oh No! We can’t find a non static ancestor

• While searching for a non static ancestor, if you reach the top of the document and haven’t found it then the <html> element is your containing block.

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Position : absolute

It is common for an author to pick an element that will serve as the containing block for the absolutely positioned element and give it a position of relative with no offsets.

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Finding the containing block

<html>

<body>

p { position: absolute; }div { position: relative; }

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Finding the containing block

<html>

<body>

p { position: absolute; }div { position: relative; }

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Position : absolute

• If top/bottom and left/right are auto then absolute elements default to their position: static values.– Look here for details

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Position : absolute

• Absolutely positioned elements are completely removed from the document flow.

• That means the space it would normally take is closed up.

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Position : absoluteHTML<p>The red fox jumps out of the <span>window</span> to go home at night</p>

CSSspan { color: red; position: static; }

The red fox jumps out of the window to go home at night.

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CSSspan { color: red; position: absolute; }

The red fox jumps out of the to go home at night.window

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CSSspan { color: red; position: absolute; top: 1em; }

The red fox jumps out of the to go home at night.window

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Position : absolute

• Absolutely positioned elements with auto values of height and width are computed to fit the contents of the element (Similar to display = inline).

• Non-replaced absolutely positioned elements can be made to fill the available space by specifying top and bottom for height and left and right for width.

• Check MDN for more details.

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Position : fixed

• Just like absolute positioning, except the containing block of the fixed element is the viewport.

• Doesn’t require lengthy search to find containing block. Goes directly to the viewport.

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CSS Position Example

• http://jsfiddle.net/blinkmacalahan/xGKZf/

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Float Property

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• The ability to wrap content around an element

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Use float for layouts

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CSS Moment of Clarity: Float

• A float box that technically has a display of block, will behave like an inline element. That is because float elements don’t exist on their own line.

• Float elements’ vertical margins won’t collapse

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Float vs Absolute positioning

• Floated elements exist outside the normal document flow. However, their space is reserved like relatively positioned elements.

• Absolutely positioned elements are removed from the flow and their space is closed.

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Clearing the Float

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• An element that has the clear property set on it will not move up adjacent to the float like the float desires, but will move itself down past the float.

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clear : both

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Clear values

• none – allows floating on both sides• left – no floated elements allowed on the left• right – no floated elements allowed on the

right• both – no floated elements allow on both

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More info on Float and Clear

• http://css-tricks.com/all-about-floats/

• http://www.cssnewbie.com/css-float-property/

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Float Exercise

• http://jsfiddle.net/blinkmacalahan/mTAkC/

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CSS Troubleshooting• Is the element in the flow or out of the flow?

– “In flow” means the element in question respects the previous elements existence

• Is the element rendering in a block display context or an inline display context?

• Check the box model properties

• Remember vertical margins can collapse for blocked adjacent elements

• Beware default browser style sheets.

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