The <!DOCTYPE> declaration is the very first thing in your document, before the <html> tag. This tag tells the browser which HTML or XHTML specification the document uses.
Note: The <!DOCTYPE> tag does not have an end tag!
HTML HTML 4.01 specifies three document types: Strict, Transitional, and Frameset.
HTML Strict DTD
Use this when you want clean markup, free of presentational clutter. Use this together with Cascading Style Sheets (CSS):
