Something about <!DOCTYPE> Back
In the first line of a HTML document, we'll always see a special tag named,
<html>. What it does is to to specify the type of this HTML, and tell browsers to read documents as a specific type.
For HTML5, we just specify like this:
<!DOCTYPE html>. That's because HTML5 does not belongs to Standard Generalized Markup Language (SGML) any more. That means that we don't need to use Document Type Definition (DTD) to include.