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

• Google Chrome
• Mozilla
• Opera
• Safari
• Internet Explorer/Edge

HTML 4.1 had three types of DOCTYPEs

1.  Strict
2. Transitional
3. Frameset