Unfortunately modifying global.config is out of the question. Any changes to global.config will affect other hosted domains and will likely break other already functioning applications. <br />
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Our configured default setting is <xhtmlConformance mode="Transitional"/>, this is the default ASP.NET configuration as recommended by Microsoft for best compatibility with legacy applications. Having said that, you should be able to override majority of default settings with in your web.config file. <br />
A quick google search will uncover more information regarding this issue http://www.google.com/search?sourceid=navclient&ie=UTF-8&rlz=1T4GZHZ_enUS229US229&q=xhtmlconformance
