Why do we use DOCTYPE in HTML?

The doctype> tag in HTML is used to indicate the version of HTML that a document is utilising. The document type declaration is what is used to describe this (DTD). The first thing you'll notice in the HTML document is the!DOCTYPE> declaration. It is inserted prior to the html tag. There is no end tag for the!DOCTYPE> tag.

Because it is required for the HTML version, if the user application experiences any problems at that time, it can be quickly determined that the problem is due to incompatibility between the HTML version and the user's browser or that certain HTML tags have been deprecated when compared to earlier versions.