# Bad value “http://www.w3.org/1999/html” for the attribute “xmlns” (only “http://www.w3.org/1999/xhtml” permitted here).
> Canonical HTML version: https://rocketvalidator.com/html-validation/bad-value-http-www-w3-org-1999-html-for-the-attribute-xmlns-only-http-www-w3-org-1999-xhtml-permitted-here
> Attribution: Rocket Validator (https://rocketvalidator.com)
> License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
The `xmlns` attribute declares the XML namespace for the document. When present on the `` element, the HTML specification requires its value to be exactly `http://www.w3.org/1999/xhtml` — no variations allowed. The URL `http://www.w3.org/1999/html` is not a recognized namespace and will be rejected by the validator. This error almost always comes from a typo: the "x" before "html" was accidentally omitted.
## Why this matters
While most browsers will still render the page in HTML mode regardless of a malformed `xmlns` value, an incorrect namespace can cause real problems in certain contexts:
- **XHTML processing**: If the document is served with an XML content type (e.g., `application/xhtml+xml`), an invalid namespace will cause XML parsers to reject or misinterpret the document.
- **Standards compliance**: Validators and automated tools flag this as an error, which can affect quality audits, accessibility checks, and CI/CD pipelines that enforce valid markup.
- **Tooling and interoperability**: XML-based tools, content management systems, and XSLT transformations rely on correct namespaces to function properly.
## How to fix it
You have two options depending on your document type:
1. **If you need the `xmlns` attribute** (e.g., for XHTML or polyglot documents): Change the value from `http://www.w3.org/1999/html` to `http://www.w3.org/1999/xhtml`.
2. **If you're writing standard HTML5**: Simply remove the `xmlns` attribute. It's optional in HTML5 and has no effect when present with the correct value — so omitting it is the cleanest approach.
## Examples
### Incorrect — misspelled namespace
The value is missing the "x" before "html":
```html
My Page
Hello, world!
```
### Fixed — correct XHTML namespace
Add the missing "x" so the value reads `xhtml`:
```html
My Page
Hello, world!
```
### Fixed — standard HTML5 without `xmlns`
If you don't need XHTML compatibility, remove the attribute altogether:
```html
My Page
Hello, world!
```
For most modern websites served as `text/html`, the third option — omitting `xmlns` entirely — is the simplest and recommended approach. Only include it if your document must also be valid XHTML or will be processed by XML tooling, and always ensure the value is exactly `http://www.w3.org/1999/xhtml`.