Something about <!DOCTYPE> Back
In the first line of a HTML document, we'll always see a special tag named, <!DOCTYPE>, before <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.
More details in https://aleen42.github.io/PersonalWiki/Programming/HTML/extra_markup/extra_markup.html#2-doctypes