W3C Nu HTML Checker API

Base URL

https://validator.w3.org/nu

Authentication

No key. A descriptive User-Agent header is mandatory; requests without one are rejected.

Rate limit

Not published as a number, but W3C throttles automated use and asks high-volume users to self-host the checker, which is available as a jar and a container image.

Pricing

Free and open source.

CORS

Enabled — callable directly from browser JavaScript

Official docs

Read the docs

Every request below was executed against the live API on 2026-08-21, and the response shown is the real body it returned — not an illustration.

curl 'https://validator.w3.org/nu/?doc=https%3A%2F%2Fexample.com&out=json'

const res = await fetch("https://validator.w3.org/nu/?doc=https%3A%2F%2Fexample.com&out=json");
if (!res.ok) throw new Error(`Request failed: ${res.status}`);
const data = await res.json();
console.log(data);

import requests

res = requests.get("https://validator.w3.org/nu/?doc=https%3A%2F%2Fexample.com&out=json", timeout=20)
res.raise_for_status()
print(res.json())

{
  "url": "https://example.com",
  "version": "26.8.20",
  "messages": [
    {
      "type": "error",
      "url": "https://example.com/",
      "message": "The character encoding was not declared. Proceeding using “windows-1252”."
    }
  ]
}

urlstring

The document that was checked. When you POST a body this is absent or synthetic, so do not rely on it in CI.

versionstring

Checker version, dated like `26.8.20`. Worth logging, because new releases add and refine checks.

messagesarray

The findings. An empty array means the document is valid; there is no separate boolean to check.

messages[].typestring

`error`, `info` or `non-document-error`. A `non-document-error` means the checker itself failed, for example it could not fetch the URL, which is very different from an invalid document.

messages[].subTypestring

Present on some messages, most usefully `warning` on `info` entries and `fatal` on parse-stopping errors.

messages[].messagestring

The human-readable description, using typographic quotes. Do not pattern-match on it; it changes between versions.

messages[].lastLine / firstColumn / lastColumn / extractinteger / string

Source location and a snippet of the offending markup, present when the message maps to a position in the document.

• Fail a CI build when a template produces invalid HTML
• Annotate pull requests with validation errors at the exact line
• Audit a whole site for markup errors by validating each URL from a sitemap
• Check generated HTML from a static site generator or email template builder
• Teach HTML with authoritative, specific error messages rather than browser guesswork