# Attribute “xmlns:og” not allowed here.
> Canonical HTML version: https://rocketvalidator.com/html-validation/attribute-xmlns-og-not-allowed-here
> Attribution: Rocket Validator (https://rocketvalidator.com)
> License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
In older XHTML documents, XML namespaces were declared using the `xmlns` attribute with a prefix, such as `xmlns:og="http://ogp.me/ns#"`. While this syntax was valid in XHTML, HTML5 does not support custom XML namespace declarations on the `` element. The W3C validator will flag any `xmlns:*` prefixed attribute (like `xmlns:og`, `xmlns:fb`, etc.) as invalid because HTML5 has a strictly defined set of allowed attributes on the `` element.
The Open Graph Protocol, originally developed by Facebook, allows web pages to become rich objects in a social graph. Major platforms like Facebook, Twitter, LinkedIn, and others rely on Open Graph `` tags to generate link previews. The official [Open Graph Protocol specification](https://ogp.me) recommends using the `prefix` attribute on the `` element instead of the `xmlns:og` namespace declaration.
The `prefix` attribute is part of the RDFa specification and is recognized by HTML5 as a valid way to declare vocabulary prefixes. By switching to `prefix="og: https://ogp.me/ns#"`, you maintain full Open Graph functionality while keeping your HTML valid.
## How to fix it
1. Locate the `` tag in your document.
2. Remove any `xmlns:og` (or similar `xmlns:*`) attributes.
3. Add or update the `prefix` attribute with the appropriate namespace declaration.
If you use multiple prefixes (e.g., Open Graph and Facebook-specific tags), you can combine them in a single `prefix` attribute, separated by a space.
## Examples
### ❌ Invalid: Using `xmlns:og` (XHTML-style namespace)
```html
My Page
Page content here.
```
This triggers the validator error: **Attribute "xmlns:og" not allowed here.**
### ✅ Valid: Using the `prefix` attribute
```html
My Page
Page content here.
```
### ✅ Valid: Multiple prefixes (Open Graph and Facebook)
If you also use Facebook-specific meta tags (like `fb:app_id`), declare both prefixes:
```html
The Rock (1996)
Page content here.
```
### ❌ Invalid: Multiple `xmlns` declarations
This older pattern with multiple namespace declarations is equally invalid in HTML5:
```html
```
Replace it with the single `prefix` attribute as shown in the examples above.