We have an API page with Mime type text/html, which is generating HTML code (not whole HTML page ) via XSL transformation. This API is called from customer web application and the API response is injected into the web page. We are using <xsl:output omit-xml-declaration="yes" method="xml"/> definition to omit XML declaration. But still a tag <!DOCTYPE html> is generated at the beginning of the API HTML output code. This problem came with the transition to XslCompiledTransform engine.