HTML Documents:
All HTML documents must start with a document type declaration: <!DOCTYPE html>.
The HTML document itself begins with <html> and ends with </html>.
The visible part of the HTML document is between <body> and </body>.
Example:-
<!DOCTYPE html>
<html>
<body><h1>My First Heading</h1>
<p>My first paragraph.</p></body>
</html>
The <!DOCTYPE> Declaration:
The <!DOCTYPE> declaration represents the document type, and helps browsers to display web pages correctly.
It must only appear once, at the top of the page (before any HTML tags).
The <!DOCTYPE> declaration is not case sensitive.
The <!DOCTYPE> declaration for HTML5 is:
For e.g<!DOCTYPE html>