Web.config Modifications
ASP.NET 2.0 introduced the ability to instruct all UI controls to render in an XHTML-compliant manner. There are three modes: transitional (default), strict, or legacy. In order to achieve XHTML compliance, we must make the following
web.config modification to every SharePoint web application:
<configuration>
<system.web>
<xhtmlConformance mode="strict"/>
</system.web>
<configuration>