Web Programming Step by Step

Lecture 6-A
Positioning

Reading: 4.4

Except where otherwise noted, the contents of this presentation are Copyright 2010 Marty Stepp and Jessica Miller.

The position property (examples) (4.4.2)

div#ad {
	position: fixed;
	right: 10%;
	top: 45%;
}

Absolute positioning

#menubar {
	position: absolute;
	left: 400px;
	top: 50px;
}

• removed from normal flow (like floating ones)
• positioned relative to the block element containing them (assuming that block also uses absolute or relative positioning)
• actual position determined by top, bottom, left, right values
• should often specify a width property as well

Relative positioning

#area2 { position: relative; }

• absolute-positioned elements are normally positioned at an offset from the corner of the overall web page
• to instead cause the absolute element to position itself relative to some other element's corner, wrap the absolute element in an element whose position is relative

Fixed positioning

• removed from normal flow (like floating ones)
• positioned relative to the browser window
  • even when the user scrolls the window, element will remain in the same place

Alignment vs. float vs. position

1. if possible, lay out an element by aligning its content
   • horizontal alignment: text-align
     • set this on a block element; it aligns the content within it (not the block element itself)
   • vertical alignment: vertical-align
     • set this on an inline element, and it aligns it vertically within its containing element
2. if alignment won't work, try floating the element
3. if floating won't work, try positioning the element
   • absolute/fixed positioning are a last resort and should not be overused

The vertical-align property

• can be top, middle, bottom, baseline (default), sub, super, text-top, text-bottom, or a length value or %
  • baseline means aligned with bottom of non-hanging letters

vertical-align example

<p style="background-color: yellow;">
<span style="vertical-align: top; border: 1px solid red;">
Don't be sad!  Turn that frown
<img src="images/sad.jpg" alt="sad" /> upside down!
<img style="vertical-align: bottom" src="images/smiley.jpg" alt="smile" />
Smiling burns calories, you know.
<img style="vertical-align: middle" src="images/puppy.jpg" alt="puppy" />
Anyway, look at this cute puppy; isn't he adorable!  So cheer up,
and have a nice day.  The End.
</span></p>

Common bug: space under image

<p style="background-color: red; padding: 0px; margin: 0px">
<img src="images/smiley.png" alt="smile" />
</p>

• red space under the image, despite padding and margin of 0
• this is because the image is vertically aligned to the baseline of the paragraph (not the same as the bottom)
• setting vertical-align to bottom fixes the problem (so does setting line-height to 0px)

Details about inline boxes

• size properties (width, height, min-width, etc.) are ignored for inline boxes
• margin-top and margin-bottom are ignored, but margin-left and margin-right are not
• the containing block box's text-align property controls horizontal position of inline boxes within it
  • text-align does not align block boxes within the page
• each inline box's vertical-align property aligns it vertically within its block box

The display property (4.4.4)

h2 { display: inline; background-color: yellow; }

• values: none, inline, block, run-in, compact, ...
• use sparingly, because it can radically alter the page layout

Displaying block elements as inline

<ul id="topmenu">
	<li>Item 1</li>
	<li>Item 2</li>
	<li>Item 3</li>
</ul>

#topmenu li {
	display: inline;
	border: 2px solid gray;
	margin-right: 1em;
}

• lists and other block elements can be displayed inline
  • flow left-to-right on same line
  • width is determined by content (block elements are 100% of page width)

The visibility property

p.secret {
	visibility: hidden;
}

• hidden elements will still take up space onscreen, but will not be shown
  • to make it not take up any space, set display to none instead
• can be used to show/hide dynamic HTML content on the page in response to events