1. Cascading Style Sheets Layout
CS 1150 Spring 2017

2. In This Lecture Controlling the position of elements
Creating site layouts
Designing for different sized screens

3. Key Concepts in Positioning Elements
CSS treats each HTML element as if it is in its own box
This box will either be a block-level box or an inline box
Block-level elements start on a new line (<h1>, <p>, <ul>, <li>)
Inline elements flow in between surrounding text (<img>, <b>, <i>)
If one block-level element sits inside another block-level element then the outer box is known as the containing or parent element
It is common to group a number of elements together inside a <div> (or other block level element).
For example, you might group together all of the elements that form the header of a site (such as the logo and main navigation)

4. Positioning Schemes
Normal flow – Every block-level element appears on a new line, causing each item to appear lower down the page than the previous one
Relative positioning – This moves an element from the position it would be in normal flow, shifting it according to the styling applied to it. This does not affect the position of surrounding elements.
Absolute positioning – This positions the element in relation to its containing element. It is taken out of normal flow, and the other elements on the page ignore the space it would have taken up.
Fixed positioning – This is a form of absolute positioning that positions the element in relation to the browser window, as opposed to the containing element.
Floating elements – Floating an element allows you to take that element out of normal flow and position it to the far left or right of a containing box

5. Normal Flow
In normal flow, each block-level element sits on top of the next one
Since this is the default way in which browsers treat HTML elements, you do not need a CSS property to indicate that elements should appear in normal flow

6. Relative Positioning
Relative positioning moves an element in relation to where it would have been in normal flow
If you set an element relative positioning, you can then style it to change its position on the page with the top, left, bottom, and right rules
position: relative;
top: 10px;
left: 100px;

7. Absolute Positioning
When the position property is given the value of absolute, the box is taken out of normal flow and no longer affects the position of other elements on the page.
The box offset properties (top, bottom, left, right) specify where the element should appear in relation to its containing element
position: absolute;
top: 10px;
left: 100px;

8. Fixed Positioning
Fixed positioning is a type of absolute positioning that requires the position property to have a value of fixed
It positions the element in relation to the browser window
When a user scrolls down the page, it stays in the exact same place
To control where the fixed position box appears in relation to the browser window, the box offset properties (top, bottom, left, right) are used

9. Overlapping Elements
When you use relative, fixed, or absolute positioning, boxes can overlap
If boxes do overlap, the elements that appear later in the HTML code sit on top of those that are earlier in the page
If you want to control which element sits on top, you can use the z-index property
Its value is anumber, and the higher the number, the closer that element is to the front
For example, an element with a z-index of 10 will appear over the top of one with a z- index of 5
z-index: 5;

10. Floating Elements
The float property allows you to take an element in normal flow and place it as far to the left or right of the containing element as possible.
Anything else that sits inside the containing element will flow around the element that is floated.
When you use the float property, you should also use the width property to indicate how wide the floated element should be. If you don’t, results can be inconsistent, but the box is likely to take up the full width of the containing element.
float: right;
width: 200px;

11. Using Float to Place Elements Side by Side
A lot of layouts place boxes next to each other. The float property is commonly used to achieve this.
To do this, you can float block-level elements to the left or right within a parent container

12. Clearing Floats
The clear property allows you to say that no element (within the same containing element) should touch the left or right hand sides of a box.
It can take the following values:
clear: left;
clear: right;
clear: both;
clear: none;

13. Parents of Floated Elements: Problem
If a containing element only contains floated elements, some browsers will treat it as if it is zero pixels tall
A CSS solution to this problem adds two CSS rules to the containing element:
overflow: auto;
width: 100%;

14. Fixed Width Layouts
Fixed width layout designs do not change size as the user increases or decreases the size of their browser window
Measurements tend to be given in pixels

15. Liquid Layouts
Liquid layout designs stretch and contract as the user increases or decreases the size of their browser window
Measurements are typically given in percentages

16. Demo Website