Why does .NET leave out style tags for non-IE browsers?

I made a label and dropdown box in ASP.NET (been coding VB.NET for a while, just branching into ASP.NET now) and I've noticed that ASP.NET leaves out style tags on browsers that are not IE.

Here's an example of code generated by the same page:
Code:
IE:
<span id="_ctl0_lblLabel_2" style="width:100px;">Non-Leavers</span>

Mozilla:
<span id="_ctl0_lblLabel_2">Non-Leavers</span>
The same happens with tables borders and various other things

I would manually change the code in the designer but it has this wonderful habit of chaging any style tags I put it to match actual properties.

I don't want to be defining widths in my style sheets otherwise they will become needlessly oversized.

Is there some kind of setting I'm missing to get these tags generated for non-IE browsers?