By using a DOCTYPE (sometimes called a DTD) on your HTML, you are telling the browser what type of HTML you plan on delivering. And the HTML5 DOCTYPE isn't even hard to remember:
<!doctype html>. But it's surprising to me how many designers don't use a DOCTYPE. This throws their customers into Quirks mode in many browsers and results in pages that are harder to predict how they will look across browsers. If you're having trouble with CSS not displaying as you like it, and you're not using a DOCTYPE, you should reconsider this choice. This article explains the current DOCTYPEs available and gives you advice on which ones are best to use and why.
Read the article: DOCTYPE List