Quote From the W3C Tips for Webmasters: Why specify a DOCTYPE? Because it defines which version of (X)HTML your document is actually using, and this is a critical piece of information needed by browsers or other tools processing the document. Since the document type is needed by web browsers, viewers and other HTML parsing tools in order to properly interpret a document as a particular version of HTML, the information provided by the <!DOCTYPE html> tag is especially significant in situations where there are no HTTP headers available or when the headers do not indicate an xHTML/XML media type such as application/xhtml+xml. Therefore, the DOCTYPE declaration is an important element in HTML documents, especially when creating polyglot HTML documents.
Ive never had problems not delaring it but you should yes