CSS: DIV containing no height on float set

Answer 1

After

 <div id='leftDiv' style='float:left;width:25%;'>
   content2
 </div>

add

     <div style="clear:both"></div>

It will solve your problem.

Answer 2

This is intentional as floats are designed for things like images in paragraphs (where multiple paragraphs can wrap around the image).

Complex Spiral has a fuller explanation as to why and Ed Elliot describes a number of approaches to making containers expand around floats. I find the overflow: hidden approach works best in most situations.

Answer 3

There is a new property introduced recently display: flow-root; Which will fix this issue without any hacks and have almost all major support

<div id='upperDiv' style='border: 1px solid #000000; display: flow-root;'>
     <div id='rightDiv' style='float:right;width:75%;'>
       content1
     </div>  
     <div id='leftDiv' style='float:left;width:25%;'>
       content2
     </div>
</div>
<div id='lowerDiv' style='height:50px;border: 1px solid #000000;margin-top:5px;'>
   content3
</div>

Answer 4

Set #upperDiv any of the following:

overflow: hidden;
width: 100%;

or

float: left;
width: 100%;

or create a rule using CSS pseudo-elements (IE8+ compatible) like this

#upperDiv:after {
  content: "";
  display: table;
  clear: both;
}

Best solution
Creating a reusable class rule like the following.

.group:after {
  content: "";
  display: table;
  clear: both;
}

Now you can apply it to anything that needs this same functionality. For example...

<div id='upperDiv' class="group" ... >

P.S. If you require IE 6/7 compatibility, checkout this post.