Question

It is my understanding that a display:block will automatically start in a new line. Why do the boxes collapse when a float:left is introduced inside a div with display:block? Here is a fiddle and below the code

#wrapper {
width:300px;
margin:0px auto;
text-align:left;
padding:15px;
border:1px solid #a39b8b;
background-color:#fff;
-moz-box-shadow: 0px 0px 10px #888;
-webkit-box-shadow: 0px 0px 10px #888;
box-shadow: 0px 0px 10px #888;}
#header { width: 100%; display: block; }
#logo { width: 100px; height: 145px; background: #fde; }
#slogan {
display: block; background: #f9ebcd; height: 35px; -moz-box-shadow: 0px 3px 10px #888;
-webkit-box-shadow: 0px 3px 10px #888; box-shadow: 0px 3px 10px #888; border: 1px solid red;
}
.left { float:left;}



<div id="wrapper">
<div id="header">
     <div id="logo"class="left">left</div>
</div>
<div id="slogan">slogan</div>

Answer 1

Floats move an element out of the normal flow of the page. If there’s nothing left in the flow inside a containing element, it will ‘collapse’.

Check the specs for more info.