In HTML, the doctype is the required "<!DOCTYPE html>" preamble found at the top of all documents. Its sole purpose is to prevent a browser from switching into so-called "quirks mode" when rendering a document; that is, the "<!DOCTYPE html>" doctype ensures that the browser makes a best-effort attempt at following the relevant specifications, rather than using a different rendering mode that is in
![!DOCTYPE](https://cdn-ak-scissors.b.st-hatena.com/image/square/aa767575b4232d312d2c5de428f0bf0955805e26/height=288;version=1;width=512/https%3A%2F%2Fdeveloper.mozilla.org%2Fmdn-social-share.cd6c4a5a.png)