CSS Positioning - class 22 slides
CA 272 - Professional
Web Site Development
CSS Positioning
Announcements
CA/CS Advising Day in Faculty Dining
Room: Wed., 11/28 from 3:00 to 6:30 p.m.
I am speaking with Prof. Lizmi’s ColdFusion class about the Web design/development business: Wed., 11/28 at 6:00 p.m., HU 313
Review - Positioning Schemes
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2.
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Static a.k.a.
‘normal flow’
Relative - offset by some value
Absolute - positioned at specified coordinates outside normal flow
Fixed - positioned to viewport
(Float - displaces text only)
Static Positioning
Normal flow not ‘positioned’
Block-level elements generate an (invisible) box
Elements ‘flow’ one after another; elements are displaced by previous elements in code
Elements cannot appear side by side
One-column layout
Relative Positioning
Element is offset by some value (relative to original position)
Element retains shape it would have had in normal flow
Normal flow preserves space originally taken up by element
Element may overlap other elements
Good for establishing positioning context
Relative Positioning
Example: relative positioned div
Exercise: relative positioned span
Open blank HTML document
Insert a few paragraphs of text
Wrap (CTRL-T) some text in <span class=“example”>…</span>
Create style rule for span with positioning type
‘relative’ and some offset values
Absolute Positioning
Element is completely removed from normal flow
(and space it occupied disappears)
Placement is specified by offset values (relative to containing block)
Always generates block-level box
Absolutely positioned elements do not interact with each other (except for stacking order)
Must know exact coordinates to position elements
See example
Fixed Positioning
Just like Absolute Positioning, except…
Elements positioned relative to viewport
(browser window) - scrolling window does not affect element
Good for frame-like layouts
IE 6 and under do not support fixed positioning
See example
Floats
(Not actually a positioning scheme - elements can be floated AND positioned)
Elements removed from normal flow…
EXCEPT floated elements displace text in normal flow (but NOTHING ELSE)
Floated elements displace other floated elements!
Margins on floats do not collapse
Use clear: left | right | both; on OTHER elements to keep them from being beside a floated element
Positioning Context - the
Containing Block
Containing block determines where positioned element is placed
For static and relative positioning, containing block is nearest block-level, table cell or inline box parent
Positioning Context
(continued)
For absolute positioning, containing block is nearest positioned element (relative, absolute or fixed)
Default positioning context is <body> element
(document window)
For fixed positioning, containing block is viewport
Offset Values
Top, Bottom, Right, Left
Units: length (px) or percentage (%)
Offset values are relative to containing block
Positive values move element inward
(relative to containing block)
Negative values move element outwards
% values are relative to height and width of containing block
Should give top or bottom AND side value
Default is top: 0; left: 0;
Stacking Order: z-index
Elements can be positioned 3-dimensionally along the z axis using zindex (only ‘positioned’ elements)
Higher values of z-index are further on top
z-index can be negative
Normal flow elements are z-index: 0;
Local stacking context
Other Layout Properties
Width/Height - important for constraining size of layout elements
>> Floated elements MUST have width specified
Display: block; - when we want inline element to behave as block element (e.g., anchor as button)
Overflow: visible | hidden | scroll
Max-width/max-height & min-width/min-height
>> very useful for liquid layouts
Unsupported by older versions of IE
Exercise: Absolute Positioning
Open a blank HTML document in DW
Go to the Layout tab in the Insert Bar
Draw some boxes in Design View (these will be absolutely positioned divs)
Go to Code View and view the CSS
position: absolute; left; top; width; height; zindex
Exercise: Absolute Positioning
(continued)
Click on one of your divs and add some text; make text a paragraph
Click on AP div ‘handle’ to select it
Delete height value in Property Inspector
You can change left, top, width, height (also drag with handle and bounding box)
Exercise: Absolute Positioning
(continued)
Set background color on divs
Drag one div to overlap another
Change z-index values (set lowest one to highest value)
Select overlapping div and Edit > Cut
Click in other div and Edit > Paste
Why is div in different place?
Exercise: Absolute Positioning
(continued)
Insert several paragraphs of text in an AP div
(e.g., from lipsum.com)
Select div and set height (to value smaller than height of text) - view change
Set overflow to hidden - view change
Set overflow to scroll; preview in browser
Web Layout Patterns
Common elements in Web layouts:
Branding & Marketing
Navigation (main nav, page nav, etc.)
Primary content (often text content)
Supplemental content (sidebars, calls to action, ads, images, etc.)
Footer content and site tools
Review: Page Layout Schemes
Fixed width - width set to fixed px value
Most designers use fixed width because easiest to design for
PRO: easier to design graphics for, can set readable line lengths of text
CON: doesn’t take advantage of screen width for users with large monitors; users with smaller monitors may have to scroll horizontally
Page Layout Dimensions
Fixed design for pixel widths - must account for browser ‘chrome’, scrollbars
800 x 600 monitor: 750px (or 760px) width
Safest width, but only 12% of users
1024 x 800 monitor: 950px+ width
Most users have this resolution now (53%)
1280 x 1024 is gaining (~23%)
Don’t worry about 640 x 480 anymore
Page Layout Schemes
Fluid/Liquid - width set to % value, adjusts to width of browser window
PRO: makes full use of browser window; user can resize
CON: may lead to very long lines of text; harder to design for
See: http://www.drexel.edu/
Page Layout Schemes
Elastic - width set in relative units (ems)
Page elements grow proportionately as user resizes text size
PRO: ideal for accessibility - users can view at whatever scale is comfortable for them
CON: very difficult to code; can lead to very large widths as user increases font size
Rarely used
Web Layouts
Examples:
www.starbucks.com
www.hueylong.com
www.newyorker.com
www.gohawaii.com
www.craigslist.org
Web Layout Patterns
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CSS Layouts with Floats and
Positioning
Review: two-column float with clearing footer
Wrapper encloses everything
Each column is floated
Footer clears floats and extends wrapper
Column backgrounds are on #wrapper
CSS Layouts with Absolute
Positioning
Can replicate layout with AP divs
BUT, must know placement of all divs
How do you place footer if variable content?
How do you create column backgrounds for unknown height?
CSS Layouts with AP continued
Absolute positioning is useful for positioning elements WITHIN layouts
E.g., masthead graphics
See “liquid layout” (linked on class homepage)
CSS Image Replacement
What if we want to replace text with a graphic?
(using CSS only)
Should leave text in HTML for accessibility and SEO
Many techniques; most common (probably) is
‘Phark’ method: text-indent: -5000px; e.g.
} h1#logo { text-indent: -5000px; background: url(logo.gif) 0 0 no-repeat; width and height as needed
Coca-Cola
CSS Image Replacement
Downsides:
If images are turned off (but CSS is on) nothing will display
Background images usually do not print
If you edit text, you’ll need to edit the graphic
Exercise: Image Replacement
Download from the class site:
Float layout (save to homework folder)
Nav button background image (save to images folder)
Join button jpeg (save to images folder)
Insert a link, “Join Today!”, at the beginning of the second paragraph of simple-pagelayout4a.html
Give this link class ‘joinButton’
Exercise: Image Replacement
(continued)
Create CSS styles (for default state and :hover) to make link a button with join.jpg as background image (button should be 100px by 100px)
(display; width; height; background; etc.)
Float button right and add left and bottom margins
Preview in browser
Need to hide text of link: text-indent: -5000px;
Preview in Firefox - click and hold down button should see dotted box off to left
Add outline: none; and preview again
Homework
Rework homework 16 to float both left and right columns, using wrapper div and clearing footer
Create a third column inside div#rtCol
Read Chapters 8 & 14 in the Visual
QuickStart Guide
