CSSE 290 Web Programming

Lecture 5: Floating and Positioning

Reading: 4.3 - 4.5

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, and Victoria Kirst. All rights reserved. Any redistribution, reproduction, transmission, or storage of part or all of the contents in any form is prohibited without the author's expressed written permission.

Otherwise note: Claude Anderson was given permission to modify the slides for CSSE 290 at Rose-Hulman by author Jessica Miller. The authors' original slides, based on Web Programming Step by Step, can be seen at http://webstepbook.com.

Some of the examples in some days' slides are from David Fisher at Rose-Hulman, who was kind enough to allow me to use them. My intention is to mark these examples with [DSF].

Announcements

• HW 2 due Thursday, March 21, at beginning of class
• A useful web site: http://caniuse.com/
• Download today's exercises. We will start with the CssLayoutBasics-master folder.

CSS specificity

• What if several different selectors could determine styles for the same HTML element? From w3.org:

A selector's specificity is calculated as follows:

• count the number of ID selectors in the selector (= a)
• count the number of class selectors, attributes selectors, and pseudo-classes in the selector (= b)
• count the number of type selectors and pseudo-elements in the selector (= c)
• ignore the universal selector

Selectors inside the negation pseudo-class are counted like any other, but the negation itself does not count as a pseudo-class.

Concatenating the three numbers a-b-c (in a number system with a large base) gives the specificity. Base 10 is usually sufficient

CSS Specificity Examples

How does this apply to the last exercise from Session 4 class?

Look at solution and see why the unexpected selector needs to be there

Chrome developer tools

• Download Session 5 exercise folder, then open box_model/three_boxes.html in Chrome or Firefox
• Right-click to inspect elements
  • To use Firefox inspector, first install FireBug

The CSS float property (reference)

• a floating element is removed from normal document flow
• "floats" above rest of document
• underlying text wraps around it if necessary

Float example

<img src="images/Keaggy.jpg" alt="Phil Keaggy" class="headericon" />
Phil Keaggy has been known for more than 40 years as one ...

img.headericon {
	float: left;
}

Phil Keaggy has been known for more than 40 years as one of the world's best electric and acoustic guitarists, even though he only has nine fingers. He has recorded about 50 solo albums, as well as eight albums with power rock trio Glass Harp. His musical styles range from folk to fingerstyle to blues-rock. One of my favorite Keaggy jams begins at time 2:55 in this YouTube video from 1985. This one shows off his amazing acoustic style (starts off slow, but really gets going!).

Float vs. alignment

• This code is in float_property/float_experiment.html.
• Edit the file to change the float values of some elements
  (float none; is one option) and watch what happens when you reload the page.

Floating content and width

I am not floating, no width set

I am floating right, no width set

I am floating right, no width set, but my text is very long so this paragraph doesn't really seem like it's floating at all, darn

I am not floating, 45% width

I am floating right, 45% width

• often floating elements should have a width property value
  • if no width is specified, other content may be unable to wrap around the floating element

The clear property

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

Clear diagram

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

Common error: container too short

<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

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

Play with various values for overflow.

Multi-column layouts

<div>
	<p>the first paragraph</p>
	<p>the second paragraph</p>
	<p>the 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

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

Resize this window and notice what happens to the "Here I am" ad.

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

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

The opacity property

body     { background-image: url("images/marty-mcfly.jpg"); background-repeat: repeat; }
p        { background-color: yellow; margin: 0; padding: 0.25em; }
p.mcfly1 { opacity: 0.75; }
p.mcfly2 { opacity: 0.50; }
p.mcfly3 { opacity: 0.25; }

Practice

• Try a few of the Layout Basics exercises. Instructions are linked from the schedule page.
• If you have time, try a few of the Flexible Layouts exercises. Instructions are linked from the schedule page.
• Work with your partner(s). Suggestion: Do it together on one student's computer, email the work to the other partners at the end of class.
• Get help from the course assistants as needed.