Why do I need a doctype? (What does it do) [duplicate]

Answer 1

From W3Schools, a doctype is "an instruction to the web browser about what version of the markup language the page is written in." (http://www.w3schools.com/tags/tag_doctype.asp)

If you do not include the doctype, the browser may assume you are using a different language than you really are, causing it to be rendered incorrectly.

Answer 2

The doctype declaration should be the very first thing in an 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 the markup language the page is written in.

The doctype declaration refers to a Document Type Definition (DTD). The DTD specifies the rules for the markup language, so that the browsers render the content correctly.

Reference