I assume that the SharePoint masterpages equal the ones in ASP.NET (MVC). Therefore this shouln't be a problem at all.
<!--[if lt IE 7 ]> <html class="ie6"> <![endif]-->
<!--[if IE 7 ]> <html class="ie7"> <![endif]-->
<!--[if IE 8 ]> <html class="ie8"> <![endif]-->
<!--[if IE 9 ]> <html class="ie9"> <![endif]-->
<!--[if (gt IE 9)|!(IE)]><!--> <html> <!--<![endif]-->
All the preceding code does is setting the HTML tag with a different CSS class on the HTML tag depending which browser is accessing the site. So that you are able to override some style sheets for any given browser (IE).
site.css
.coloredBackground
{
background-color: #FF0000; //red
}
.ie6 .coloredBackground
{
background-color: #00FF00; //green
}
.ie8 .coloredBackground
{
background-color: #0000FF; //blue
}
In this example users browsing with Firefox, Opera, Safari, IE7,9,10 will see a red background. For IE6 the background color gets overridden with green and in IE8 with blue.
Your CSS registration would look like the following:
<SharePoint:CSSRegistration Name="site.css" runat="server" />
As you can see there is no need to set the ConditionalExpression anymore in the CSS registration, because you are already switching the used style sheet by setting a specific class on the HTML element.
Update:
Another possibility would be to include another style sheet file depending on the browser version using the ConditionalExpression property on the SharePoint aspx control.
<SharePoint:CSSRegistration Name="ie6.css" ConditionalExpression="lt IE 7" runat="server" />
<SharePoint:CSSRegistration Name="ie7.css" ConditionalExpression="IE 7" runat="server" />
<SharePoint:CSSRegistration Name="ie8.css" ConditionalExpression="IE 8" runat="server" />
<SharePoint:CSSRegistration Name="ie9.css" ConditionalExpression="IE 9" runat="server" />
The downside is that you may get css priority issues because the .ie* class is missing on the html element and therefore doesn't overrule the .class-to-override-if-specific-ie-version. You could solve this by using the !important rule in the ie specific style sheet files.
