even with comlpiant browsers, valid html and valid css you can still get
display difference between browsers!
take the following (abrevieted) example
<html>
<head>
<style>
#big {width : 100px; height :100px; background-color : red;}
#small {width : 50px; height: 50px; background-color : blue;}
</style>
</head>
<body>
<div id="small">
a small div containing
<div id="big">
a big div
</div>
</div>
</body>
</html>
now there is nothing invalid about either the CSS or the HTML above
But it is clearly bad coding - the big div can't fit inside the small
one - and the browser has to either increase the size of the small div,
decrease the big one, break the containment ...