What is DOCTYPE and why is it necessary?

DOCTYPE (short for Document Type Declaration) is a document type declaration that is placed at the very beginning of an HTML document, before the <html> tag.

It is necessary for:

Activating Standards Mode in browsers: Without DOCTYPE, browsers may switch to Quirks Mode, where they try to emulate the behavior of older browsers, which can lead to incorrect rendering of the webpage due to non-standard handling of CSS and HTML. A proper DOCTYPE forces the browser to follow modern standards.
Document validation: DOCTYPE references a DTD (Document Type Definition) or schema that defines the rules for the structure and syntax of a specific version of HTML or XML. Validators can use this information to check if the document complies with these rules.
Specifying the HTML/XHTML version: DOCTYPE explicitly indicates which version of HTML or XHTML the document conforms to.

Modern HTML5 DOCTYPE is very simple:

<!DOCTYPE html>

For older versions of HTML and XHTML, DOCTYPEs were more complex and included references to DTDs, for example:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">