Getting different font size with same css file in IE - asp.net

Question

I have created a couple of ASP.Net (aspx) pages which are supposed to run with existing ASP pages in a website.
Both ASP and ASP.Net pages use the same css file. In both the pages, there are headings which are styled using tag from css file.
The problem is that headings in .asp and .aspx pages are rendering in different size in Internet Explorer. It works fine in FF and Chrome.
Please suggest some workaround for this, as I have to make my .aspx pages similar in look and feel to the existing .asp pages.
Thanks in advance!
Updated:
I can paste a part of css file here, since it is a large file:
TH, FONT, TD, P, B, I, STRONG, U, EM, BLOCKQUOTE, LI, OL, CAPTION, DL, DT, DD,select,pre {font-size:11px;font-family:Arial, Helvetica, sans-serif;}
body{
font-family: Arial, Helvetica, sans-serif;
color: #000000;
line-height: 1.166;
margin: 0px;
padding: 0px;
}
h2{
font: bold 150% Arial, Helvetica, sans-serif;
color: #000000;
margin: 0px;
padding: 0px;
}
The aspx page (heading part) looks like this:
<table style="left: 35px; position: static; top: 231px">
<tr>
<td style="height: 85px; width: 2087px;">
<h2><asp:Localize id="lblHeading" runat="server" Text="Welcome to XYZ" meta:resourcekey="lblHeading"></asp:Localize>
</h2>
</td>
</tr></table>
ASP page has a simple tag:
<h2>Welcome to XYZ</h2>
Updated:
The weirdest thing happened. I tried stripping down the css file and started adding each style one by one, and refreshing the page to check which styles are getting applied to the heading. I noticed that if I remove the first line of css (keeping all the other styles intact):
<STYLE type="text/css">
then the headings look same in both asp and aspx pages. As soon as I add the above line in css file, the heading in aspx page becomes bigger.
Any comments as to why this is happening?

Answer 1

Make sure you are using the proper and the same Doctype at the top of the ASP and ASPX pages. You can also use the developer tools in FF and in IE to make sure that the same styles are applied to headings on ASP and ASPX pages in both browsers. What version of IE are you using?

Answer 2

You might be interested in section about w3schools recommendation for font-size
The suggested way, as I understand is to declare in body:
body{
font-size: 100%; // which default is 16px
}
and everywhere else use em (instead of % or px), as explained in the link.
So in your header you should not have 150%, but instead DesiredSizeInPx divded by 16, for example if you want to have 24px you write 24/16em, which is 1.5em

Answer 3

The problem could be that two different styles are being applied.
The ASP page is getting the straight h2 style
h2{
font: bold 150% Arial, Helvetica, sans-serif; etc.
Whereas the ASPX might be getting another style
id="lblHeading"
Check to see if #lblHeading is giving a different size.