---
title: "The HTML doctype"
category: foundations
status: required
url: https://specification.website/spec/foundations/doctype/
updated: "2026-05-29"
sources:
- title: "HTML Living Standard — The DOCTYPE"
url: "https://html.spec.whatwg.org/multipage/syntax.html#the-doctype"
publisher: "WHATWG"
- title: "MDN — Doctype"
url: "https://developer.mozilla.org/en-US/docs/Glossary/Doctype"
publisher: "MDN"
- title: "MDN — Quirks Mode and Standards Mode"
url: "https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode"
publisher: "MDN"
source_repo: https://github.com/jdevalk/specification.website
licence: CC-BY-4.0
---
# The HTML doctype
> Every HTML document must start with as its first line. This opts the browser into standards mode; without it, you get quirks mode and broken layout.
## What it is
The doctype is a short declaration at the very top of an HTML document that tells the browser which rendering mode to use. In modern HTML, there is exactly one correct form:
```html
```
It is case-insensitive, so `` is equally valid. It must be the first thing in the document, before ``, with no whitespace, comments, or byte-order mark trickery in front of it.
## Why it matters
Without a doctype, browsers fall back to **quirks mode** — a compatibility layer that emulates the buggy behaviour of browsers from the late 1990s. In quirks mode:
- The CSS box model changes (widths include padding and border, like the old IE5 model).
- Inline elements behave differently around whitespace.
- Many modern CSS features are unreliable or disabled.
- Table cell heights, image alignment, and font sizing all shift.
With `` you get **standards mode**, where the browser follows current specifications. There is also a "limited-quirks" (almost-standards) mode triggered by some legacy doctypes, but you should never need it.
In short: one missing line at the top of the document silently changes how every CSS rule on the page is interpreted. It is the cheapest correctness fix on the web.
## How to implement
Put the doctype on line one of every HTML response. No XML prolog, no comment, no blank line:
```html
Page title
...
```
The old HTML 4 and XHTML doctypes (`HTML 4.01 Transitional`, `XHTML 1.0 Strict`, etc.) are obsolete. Replace them with ``. The short form is part of the HTML Living Standard and works in every browser back to IE6.
For XML-serialised HTML (XHTML served as `application/xhtml+xml`), the doctype is optional, but the document must still parse as XML. Almost no public sites need this — serve HTML.
## Common mistakes
- A blank line, comment, or BOM before ``. Anything before the doctype can trigger quirks mode.
- Using a legacy HTML 4 or XHTML doctype copied from a 2005 tutorial.
- Sending the doctype only on some pages — error pages, print views, and embedded iframes need it too.
- Letting a templating engine strip it during minification.
## Verification
- View source on the page. The very first bytes must be ``.
- In DevTools, run `document.compatMode` in the console. It should return `"CSS1Compat"` (standards mode), not `"BackCompat"` (quirks).
- Check error pages (404, 500), redirect destinations, and any HTML fragments returned by APIs.