CSS Positioning

Objectives

Students should, at the end of the lesson, be able to:

Establish spacing inside and outside of elements using margin and padding.
Explain the difference between different types of distance measurement in a web page, including 'px', '%', and 'em'.
Use 'float' and 'clear' to stack elements alongside each other.
Explain the difference between 'static', 'relative', 'absolute', and 'fixed' positioning.

Overview

So far, we've mostly talked about using CSS for styling our page - adding colors, fonts, etc. Next, we'll be examining how CSS can be used to control a webpage' layout. Back in the 90s, layout was accomplished using tables (<table>), which had rows (<tr>) and row subdivisions (<td>). However, this was problematic for several reasons.

Layout was hard-coded into the page - it couldn't adjust easily.
As a result of (1), keeping layout consistent between multiple pages was tedious.
Nesting tables within tables quickly became a nightmare; how could you tell apart the <tr> of one level from the <td> of another?
It wasn't very semantic - our markup would always say 'table', even though our content was typically not a table.

Using CSS to control our layout addressed all of these issues. What's more, it effectively abstracted away the layout of our page from the content of our page.

Basic CSS Positioning

Dimensions

In addition to setting an element's height and width,elements have three other properties that can be used to explicitly control spacing with other elements.:
1. 'Border' sets a perimeter around an element. In addition to specifying a color and a particular type of border, you can also specify a thickness.
2. 'Margin' specifies spacing between the outside of an element's border and any adjacent elements.
3. 'Padding' specifies spacing between the inside of an element's border and the contents of that element (which includes height and width!)
All of these properties, including height and width, can be specified in the following terms
- px : number of pixels
- % : ties an element's dimensions to the size of the element that contains it ("parent"). As a value of height, % refers to a fraction of the parent's height, but as a value of everything else, % refers to a fraction of the parent's width value.
- em : ties dimensions to font size - one em is the width of the letter 'm'. For all dimensions except font-size, em will refer to the font size of the element; as a value for font-size, em refers to the font size of the parent

Float and Clear

The problem with block elements is that they always stack vertically, never side by side. This is because each block element has a 'new-line' built into it.
This can be circumvented using the float property; floated elements pop out of the normal flow of a document and stack up in the upper left or upper right corner of their container, while the rest of the content wraps around them.
As more elements get floated, they continue stacking up alongside the first floated element from left to right (if floated left; if floated right, elements stack up from right to left.)
Setting float to none will cause that element not to be floated, but any inline elements it holds will still wrap around any floated elements.
To make something fall beneath a set of floated elements, rather then wrapping around it, you can use the clear attribute; set clear to left to clear a left float, right to clear a right float, or both to clear either kind of float.

Lab :: Basic CSS Positioning

In your squads, create your own look-alikes for the following websites using what you've learned about so far about CSS positioning, including margin, padding, float and clear.

http://nytimes.com
http://en.wikipedia.org/wiki/Main_Page
http://reddit.com Start this process by drawing a box model - a guess as to how the elements on the page are broken out into <div>s (or equivalent elments).

Ordinarily, elements expand to hold their containers. However, floated elements are excluded from this, so floating an element may lead to its container's height shrinking down to nothing. Keep this in mind when using floats!

Advanced CSS Positioning

All of the rules that you've learn so far are based on one paradigm of positioning, called 'static' positioning. However, it's possible to change this paradigm and employ a different approach for positining elements using the position attribute.
relative positioning allows you to define where an element should go based on where it would go if it was statically positioned. For instance, changing a static element's positioning to the following
```
position: relative;
  left: 10 px
```

would shift it to the right by 10 pixels.

absolute positioning allows you to define the position of an element with respect to a 'relative-ly' positioned parent element that contains it (or, if no such parent exists, with respect to the body). Giving an element the following positioning will place it ten pixels to the right and 20 pixels down from the top-left corner of its parent element/the body.
```
position: absolute;
  left: 10px;
  top: 20px;
```
fixed positioning defines the position of an element with respect to the view window, essentially 'fixing' its position on the screen. Fixed positioning is frequently used in parallax scrolling.

position: fixed;
  left: 130px;

Lab :: Advanced CSS Positioning

As in the previous exercise, work in your squads to create your own look-alike for the following websites; however, this time, try to use all four types of positioning at least once.

HoldenMills / html-css-layout-positioning