DOCTYPE in HTML

The DOCTYPE declaration is the first thing in your HTML document and it will be used before the HTML tag.
The DOCTYPE declaration is not an HTML tag.
<!DOCTYPE> declaration is not a HTML tag.
<!DOCTYPE> tells or instruct the web browser that what html version is using.
It must be the first thing on the web page before the tag HTML is writing.
The browser will understand what type of HTML document you are using if you add DOCTYPE to the web page.
It just lets the browser know that the document type is HTML5. It defines that the document is in HTML 5 not and ordinary HTML5.
The DOCTYPE Declaration is not case sensitive.
The DOCTYPE declaration for HTML5 is simplified.
<!DOCTYPE HTML>

Browsers Support for DOCTYPE