auto is the default value for width and height.

  

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<style type="text/css">
.container{
    width: 800px;
    height: 800px;
    background: pink;
}
* .box {
  float: right;
  overflow: auto;
  visibility: visible;
  width: auto;
  height: 100px;
  margin: 10px;
  padding: 10px;
  background: red;
}

* .small {
    float: left;
    right: 200px;

    background: yellow;  
}
</style>
</head>
<body>
  <div class="container"> 
    <div class="small">this is a test. <BR/>this is a test. this is a test. this is a test. </div> 
    <div class="box">this is a test</div> 
  </div> 
</body>
</html>

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1.	Box Dimensions
2.	Box Model defines properties of: display, width, height, padding, border, margin, background, overflow, and visibility.
3.	Spacing: indentation, margins, padding, letter spacing, word spacing, text justification, and line spacing
4.	Box Model Basics
5.	Understanding the Box Model
6.	The margin Properties
7.	The Padding Properties
8.	Box model for child and parent relationship
9.	Inner box
10.	Inner box with DIV
11.	Inner box padding and border
12.	Left text alignment for inner box
13.	Box model with div
14.	Width setting
15.	block multiline inline box model with margin and padding
16.	Float left in a container
17.	Float inside
18.	Simple float clearing in Mozilla
19.	Setting box dimensions and aligning text
20.	Floating elements and element boxes
21.	Setting borders of an element
22.	Block Box
23.	Table Box
24.	border and padding shrink the inner box of stretched absolute boxes.
25.	A percentage of the height of its container.
26.	A percentage of the width of its container.
27.	Style sheet for boxModel illustration

auto is the default value for width and height. : Box model Basics « Style Basics « HTML / CSS

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