What happens when you remove doctype from the HTML page

What happens when you remove doctype from the HTML page

Without a Doctype:
The browser enters Quirks mode and tries to deal with your code as if it was written in the late 90’s. This means they will imitate many bugs that existed in the old browsers. It is not consistent on how to do this (so you will get significantly different results, for example, from Firefox to Internet Explorer).

The purpose of DOCTYPE is to tell the browser what type of HTML you are writing. It is not valid to omit the DOCTYPE. There is no “Standard” format. The browser will just try to parse HTML as best it can. But not all elements will be displayed correctly. DOCTYPE is a required part of all HTML documents.
In HTML5 the tag used is: <! DOCTYPE html>