Web Programming Step by Step

Lecture 5
Floating and Positioning

Reading: 4.3 - 4.5

Except where otherwise noted, the contents of this presentation are Copyright 2009 Marty Stepp and Jessica Miller.

The CSS float property (reference) (4.3.1)

img.headericon {
	float: right;    width: 130px;
}

Borat Sagdiyev (born July 30, 1972) is a fictional Kazakhstani journalist played by British-Jewish comedian Sacha Baron Cohen. He is the main character portrayed in the controversial and successful film Borat: Cultural Learnings of America for Make Benefit Glorious ...

• removed from normal document flow; underlying text wraps around as necessary

Floating elements diagram

Common float bug: missing width

I am not floating, no width

I am floating right, no width

I am not floating, 45% width

I am floating right, 45% width

• often floating block elements must have a width property value
  • if no width is specified, the floating element may occupy 100% of the page width, so no content can wrap around it

The clear property (4.3.2)

p { background-color: fuchsia; }
h2 { clear: right; background-color: yellow; }

Clear diagram

div#sidebar { float: right; }
p { clear: right; }

Common error: container too short (4.3.3)

<p><img src="images/homestar_runner.png" alt="homestar runner" />
	Homestar Runner is a Flash animated Internet cartoon.
	It mixes surreal humour with ....</p>

p { border: 2px dashed black; }
img { float: right; }

• We want the p containing the image to extend downward so that its border encloses the entire image

The overflow property (4.3.3)

p { border: 2px dashed black;
    overflow: hidden; }

Multi-column layouts (4.3.4)

<div>
	<p>first paragraph</p>
	<p>second paragraph</p>
	<p>third paragraph</p>
	Some other text that is important
</div>

p { float: right; width: 20%; margin: 0.5em; 
    border: 2px solid black; }
div { border: 3px dotted green; overflow: hidden; }

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

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 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)