Every HTML document you create requires a document-type declaration--DOCTYPE. This declaration begins the HTML document and tells a validator which version of HTML to use when checking its syntax.
GoLive 6 offers an easy way to add or change the DOCTYPE for a page you're working on. Just click on the document window menu in the upper-right corner and select the appropriate DOCTYPE from the Doctype dropdown menu.
Adding the DOCTYPE in Adobe GoLive 6 is as simple as selecting it from a dropdown menu.