JChung2006 wrote:
 Both XHTML 1.0 Transitional and XHTML 1.1 XSD's reference the same "buttonContentElements" group, but neither has it defined. Your version of the XHTML 1.1 XSD is also missing the "buttonContentElements" group definition.

Is it in one of the includes?

<xsd:include schemaLocation="CommonHTMLTypes.xsd" />
<xsd:include schemaLocation="I18Languages.xsd" />