What is a DOCTYPE and why is it used?

A DOCTYPE is a declaration that tells the browser which version of HTML the page is written in.
In HTML5, it looks like this:

Why is it used?

• It makes sure the browser renders the page in standards mode, not in old “quirks mode”.
• This ensures your page displays consistently across different browsers.
• Helps the browser understand how to interpret the HTML.

In short:
The DOCTYPE is a simple instruction to the browser to use modern, standard HTML rules when rendering the webpage.