Internet Explorer bug with assigning CSS classes

Wednesday, Nov 10, 2010 3 minute read Tags: css javascript internet-explorer web
Hey, thanks for the interest in this post, but just letting you know that it is over 3 years old, so the content in here may not be accurate.

Today I was fixing a problem on a site in which some background images weren’t showing up on certain elements in Internet Explorer but they were showing up under Firefox and Chrome.

The page is quite a complex one which does a lot of client-side building of DOM elements so I started digging around in there, finding the section which was creating the element.

The code was very simple, all it did was create a <span /> tag, assign some CSS classes to it and eventually add it to the DOM. Nothing overly complex about it but it was breaking none-the-less.

So I fired up the (lovely…) IE7 (yes, I’m on a SOE with IE7) and inspected the DOM. Sure enough the element was in the DOM, but when I looked at the applied styles in the inspector I noticed that the styles from the CSS class did not exist. According to the DOM inspector the CSS class was applied, just none of the rules were. I started to be confused, I tried manipulating the stylesheet, adding some more sizing to the element, but nothing caused the rules to be applied. But if I started playing in the DOM inspector I could influence it but only with what I was custom adding.

After scratching my head for a while I took another look at the element creation process, and then I noticed something very strange…

span.setAttribute('class', 'someClass');

The developer who wrote the JavaScript was using setAttribute method on the DOM element to set the CSS class, not the className property. I’ve never done it via the method, so I changed it to use the property and vola the CSS class was applied!

I then created a very simple little piece of HTML to test with to ensure it wasn’t something more of a problem from the overall page, but it always fails in IE, here’s my sample code:

		<title>IE CSS assignment testing</title>
		<style type="text/css">
			.c { background-color:#ff0000;}
			.s { padding-top:10px; background-color:#00ff00;}
		<div id="s"></div>
		<script type="text/javascript">
			var txt = document.createElement('span');
			txt.innerHTML = "Hello World";
			txt.setAttribute('class', 'c');
			var s = document.getElementById('s');			
			s.setAttribute('class', 's');

Save that as a HTML file and open it in IE7, IE9 Beta (I don’t have 8 or 6 on a machine), Firefox 3.6.11 and Chrome 8. In both the IEs I tested the background colour & padding is not applied, despite the inspector saying that the element has the classes applied to it.

I’ll be reporting this as a bug in IE shortly, but of future note to developers use element.className not elemnet.setAttribute for CSS class assignment!