Specifying the Document Type
Here, you will learn how to specify the document type. So whenever you start creating an HTML5 document, the first line of the document should be: <!DOCTYPE html>
The DOCTYPE tag always begins with an exclamation point and is always placed at the beginning of the document.
Most of the HTML tags are not case-sensitive, but the word DOCTYPE should always be written in uppercase.
Using the DOCTYPE tag is like signing a contract. It is an optional tag. Using DOCTYPE tag means that you are using HTML5 so every time when the browser sees this tag <!DOCTYPE html>, it assumes that the user is using the HTML5 Script.