Validación HTML: Attribute with the local name “xmlns:(fb|og)” is not serializable as XML 1.0

Acerca de este problema HTML

Understanding the Issue

In the early days of the Open Graph Protocol (used by Facebook, LinkedIn, and other platforms to parse shared links), developers were instructed to add XML namespace declarations like xmlns:og and xmlns:fb to the <html> element. This was borrowed from XHTML conventions where custom prefixed attributes required namespace bindings.

However, modern HTML5 documents are not XML. The HTML5 parser does not support arbitrary namespace declarations in the way XHTML/XML does. Attributes with colons in their local names (like xmlns:og) are not serializable as XML 1.0, meaning they cannot be reliably round-tripped between HTML and XML parsers. The W3C validator flags this because these attributes violate the HTML serialization rules.

The good news is that these namespace declarations have never been required for Open Graph tags to work. Social media crawlers (Facebook’s, Twitter’s, LinkedIn’s, etc.) parse <meta> tags based on the property attribute value directly — they don’t rely on XML namespace resolution. Browsers also ignore these attributes entirely, so removing them has zero impact on functionality.

How to Fix It

Locate your <html> opening tag.
Remove any xmlns:og, xmlns:fb, or similar namespace attributes.
Keep your Open Graph <meta> tags exactly as they are — they will continue to work.

Examples

❌ Incorrect: Namespace attributes on the `<html>` element

<!DOCTYPE html>
<html lang="en" xmlns:og="http://ogp.me/ns#" xmlns:fb="http://www.facebook.com/2008/fbml">
  <head>
    <title>My Page</title>
    <meta property="og:title" content="My Page Title">
    <meta property="og:description" content="A description of my page.">
    <meta property="og:image" content="https://example.com/image.jpg">
  </head>
  <body>
    <p>Hello, world!</p>
  </body>
</html>

This triggers the validator warning: Attribute with the local name “xmlns:og” is not serializable as XML 1.0 (and similarly for xmlns:fb).

✅ Correct: Clean `<html>` element without namespace attributes

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>My Page</title>
    <meta property="og:title" content="My Page Title">
    <meta property="og:description" content="A description of my page.">
    <meta property="og:image" content="https://example.com/image.jpg">
  </head>
  <body>
    <p>Hello, world!</p>
  </body>
</html>

The Open Graph <meta> tags remain unchanged and will continue to be recognized by Facebook, LinkedIn, and all other platforms that support the Open Graph Protocol. The only change is removing the unnecessary xmlns:og and xmlns:fb attributes from the <html> tag.

Common Variations

You may also encounter other namespace prefixes like xmlns:article, xmlns:product, or xmlns:music. These are all unnecessary in HTML5 and should be removed:

<!-- ❌ Remove all of these -->

<html lang="en" xmlns:og="http://ogp.me/ns#" xmlns:article="http://ogp.me/ns/article#" xmlns:fb="http://www.facebook.com/2008/fbml">

<!-- ✅ Keep it simple -->

<html lang="en">

Note that the standard xmlns attribute (without a colon prefix) is allowed on the <html> element in HTML5 if its value is exactly http://www.w3.org/1999/xhtml. However, even this is unnecessary and can be omitted. The prefixed forms like xmlns:og are never valid in HTML5.