Laying Out Elements with CSS

Objectives
Assess the CSS box model
Set element width and borders
Set margins and padding
Align elements with float
Control page flow with clear
HTML5 and CSS3 – Illustrated, 2nd Edition 2

Objectives (continued)
Implement fixed positioning
Implement relative positioning
Control stacking order
Implement absolute positioning

Assess the CSS Box Model
Box model: used by CSS to represent
characteristics of every web page
element
 Treats element as rectangular box
border: border surrounding element
margin: space from border to
neighboring/parent element
padding: space between border and
element content

(continued)
The CSS box model

(continued)
Box model properties

(continued)
CSS units of measure
 Relative units: ems, percent, rems
 Absolute units: pixels
Commonly used CSS units of
measure

(continued)
Size of padding, margin, and
border increase the amount of space
occupied by an element
 Dimensions of these properties not
included in specified width / height
• Specified width and height refer only to the
content of an element
• To calculate add the total space, add the
width value to the padding, margin, and
border values

(continued)
Calculating web page space occupied
by an element

(continued)
Collapse: When bottom margin of one
element is adjacent to top margin of
another, the margins combine to the
size of the larger of the two
 Affects element size planning
Separate property for each side of
padding and margin, e.g.
 padding-top: 2px;
 margin-right: 1em;

Set Element Width
and Borders
Margin, padding, and border are
shorthand properties
 Assign values to multiple CSS properties
 Assign values to all four sides of an
element
 Assign these values using a single
declaration
Can set width, style, or color of any or
all sides of an element

Set Element Width
and Borders (continued)
Specific properties for the box model
Reset rule: resets one or more
common properties to a common
baseline, e.g.
 border: 0

Set Element Width
Code with width and border
declarations

Set Element Width
width and border declarations
rendered in a browser and developer
tools opened to show box model

Set Margins and Padding
Use the shorthand property to assign
multiple values
 Separate each value with a space
 Meaning of different number of values
Assign auto to left and right margins
to center the element horizontally

Set Margins and Padding
(continued)
Example of padding using three
values and margin using two values
Example of margin set to auto

Align Elements with float
Page flow: the order of elements in the
HTML document
 User agents render HTML top to bottom
 Easy to create layouts with elements
stacked vertically
Use float, clear, and width
properties to create columns of text
and graphics parallel to each other
 Use width property to assign width to
each of the columns

(continued)
Example of float applied to element
Use float to create multicolumn
layoutsHTML5 and CSS3 – Illustrated, 2nd Edition 18

(continued)
Code with float and result in browser

Control Page Flow
with clear
float gives basic control over layout
clear gives more precise control
 Prevents floated elements from being
displayed to the left, right, or on the side
of another element
clear property values

Control Page Flow with clear
(continued)
Example of clear property

Control Page Flow with clear
(continued)
Code including clear property and
how it is rendered in a browser

Implement Fixed
Positioning
fixed positioning keeps an element
in the same location, even when the
page is scrolled
Use the position property with the
value of fixed
Then specify
 Horizontal position using left or right
 Vertical positing using top or bottom

Implement Fixed Positioning
(continued)
Commonly used position properties
Properties and values for fixed
positioning

(continued)
Example showing fixed positioning

(continued)
Code including fixed value and how
it is rendered in a browser

Implement Relative
Positioning
Relative positioning: adjusting default
position of an element
 Preserves space allotted to element in
default page flow
 Set position property to relative
 Style exact location using the properties
•left and right for horizontal positioning
•top and bottom for vertical positioning

Implement Relative
Positioning (continued)
CSS properties and values for relative
positioning

Implement Relative
Implementing relative positioning and
page flow

Implement Relative
Code using relative positioning and
how it is rendered in a browser

Control stacking order
Stacking elements: positioning
elements so that they can overlap
 Additional possibilities for layouts
 Applies only to positioned element
 Elements placed in new layer
 Requires careful planning
Stacking order controlled by values
assigned to z-index property

Control Stacking Order
(continued)
Stacking positioned elements

Control Stacking Order
(continued)
Code to stack and how it is
rendered in a browser

Implement Absolute
Positioning
Absolute positioning: takes element
out of page flow entirely
 Allows other elements to flow into space
element would have occupied
 Set position property to absolute
 Style exact location using left, right,
top, and bottom properties
 Location is calculated relative to closest
ancestor element that has position
applied to it

Implement Absolute
Absolute positioning and page flow
Properties and values

Implement Absolute
Code to absolutely position an
element and the result in a browser

Summary
Box model: used by CSS to represent
characteristics of elements
 Treats element as rectangular box
 Defines border, margin and padding
Margins of adjacent elements can
collapse into one margin
Specify border width, style,
and/or color of any or all sides

Summary (continued)
Use multiple values to assign margin
and padding values to different sides
Use float to change the default
page flow
Use clear to create a more precise
layout
Use float, clear and width to
create multicolumn layouts

Summary (continued)
Fixed positioning: keeps an element in
the same position, even when the
page is scrolled
Relative positioning: adjusting location
of element relative to its page flow
location using position property
 Space preserved

Summary (continued)
Use z-index to create a stacking
order for elements
 Stacked elements are placed in new
layers
Absolute positioning: takes elements
out of page flow entirely
 Other elements reflow into empty space

Laying Out Elements with CSS

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