The document discusses the importance of including the <!DOCTYPE> declaration at the top of HTML documents to instruct browsers on the document type and ensure standard rendering. The main document type options are HTML 4.01, HTML 5, and XHTML 1.0, with their respective doctype declarations. Omitting the doctype can trigger quirks mode in browsers and result in inconsistent styling across browsers. Developers are advised to always include the doctype as the very first line of code in HTML documents.