The DOCTYPE declaration should be the very first thing in any HTML document, before you insert your tag. It is important to remember that DOCTYPE declaration is not an HTML tag; it is an instruction to the web browser about what version of the markup language your web page is written in.

By using a certain DOCTYPE (strict or transitional) you claim to have correctly implemented a certain (X)HTML flavor:

There are various DOCTYPE that you can use with your code. Following list is a quick summary of commonly used DOCTYPE declarations.
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