The correct appearance and web documents handling cannot be guaranteed simply by applying strict, error-free markup. Web server configuration has a significant impact on web site appearance, operation, and behavior. Documents should be served with the proper media type and character encoding. Content negotiation can be used to serve various document versions to browsers supporting the corresponding media types. XHTML documents can be served as either HTML or XML, but there is a huge difference in processing. XML files are processed by XML parsers that are far more error-sensitive than SGML parsers. XHTML served as XML involves the risk that the document cannot be rendered at all. On the other hand, backward-compatible serving cannot use the benefits of strict XML markup. There are several aspects of sending HTML and XHTML from the server to the rendering engine, all of which should be considered to achieve proper settings.