The <!DOCTYPE> declaration must be the very first thing in your HTML document, before the tag. The <!DOCTYPE> declaration is not an HTML tag; it is an instruction to the web browser about what version of HTML the page is written in. A lot of IDEs allow users to leave this out and default to a certain HTML style, but leaving it out does pose a potential threat in browser compatibility and the use of older versions of HTML. For example: new features & tags in HTML5 such as <article>,<footer>, <header>,<nav>, <section> may not be supported if the <!DOCTYPE> is not declared.