HTML Guides for xhtml

The xmlns:web attribute is an XML namespace declaration for a web prefix, and HTML doesn't allow prefixed namespace declarations on any element.

In XML, xmlns:web="http://example.com/schemas/web/" binds the prefix web to a namespace, so names like web:result can appear in the document. The URI identifies the namespace, and web is only a short alias the template's author picked. On the root <html> tag, the declaration is typically left over from a template first written as XHTML, where it sat next to xmlns="http://www.w3.org/1999/xhtml" and xml:lang.

The HTML parser doesn't process namespace declarations. It keeps xmlns:web as a plain attribute with a colon in its name, and it lowercases the name, so the validator reports xmlns:web even when the source says xmlns:Web. The only namespace attribute HTML accepts is xmlns, with the exact value http://www.w3.org/1999/xhtml. That one is allowed but does nothing in an HTML page, so you can keep it or drop it.

Delete xmlns:web from the tag. Browsers don't act on the declaration, so removing it doesn't change how the page looks or works. If the markup also has web: attributes, the validator reports each of them as not allowed. Move that data into data-* attributes, or remove it if nothing reads it.

Invalid example

<!DOCTYPE html>

<html lang="en" xmlns="http://www.w3.org/1999/xhtml" xmlns:Web="http://example.com/schemas/web/">

<head>

<title>Search results</title>

</head>

<body>

<p>Results for your search.</p>

</body>

</html>

Valid example

<!DOCTYPE html>

<html lang="en">

<head>

<title>Search results</title>

</head>

<body>

<p>Results for your search.</p>

</body>

</html>

The xmlns attribute has the XHTML namespace written with https://, but the only allowed value is http://www.w3.org/1999/xhtml.

A namespace name looks like a web address, yet no browser ever loads it. Parsers treat it as a fixed name and compare it character by character, so https://www.w3.org/1999/xhtml names a different namespace than the HTML one. The https usually slips in when a site moves to HTTPS. A search and replace across the templates, or a plugin that upgrades every http:// string, changes the namespace along with the real links. The SVG and MathML namespaces keep http:// too.

In a page served as text/html, the parser puts every element in the HTML namespace by itself, and the HTML standard says xmlns has no effect there, which is why the page still renders. The value matters once the same markup is parsed as XML, for example when it's served as application/xhtml+xml or run through XSLT. Its elements then land in an unknown namespace, and browsers and XML tools no longer treat them as HTML.

Change https back to http. In a page served as HTML you can delete the attribute instead, and keep it only if the same file also has to work as XHTML. If an HTTPS rewrite caused the change, exclude namespace names from it so the next run doesn't break them again.

Invalid example

<!DOCTYPE html>

<html xmlns="https://www.w3.org/1999/xhtml" lang="en">

<head>

<title>Products</title>

</head>

<body>

<p>Our product range.</p>

</body>

</html>

Valid example

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml" lang="en">

<head>

<title>Products</title>

</head>

<body>

<p>Our product range.</p>

</body>

</html>

The xml:lang attribute is a holdover from XHTML, where it was the standard way to declare the language of an element. In HTML5 (the text/html serialization), the lang attribute is the proper way to specify language. The HTML specification allows xml:lang for compatibility purposes, but only if it is accompanied by a lang attribute with an identical value. If xml:lang appears alone, or if its value doesn't match the lang attribute, the document is non-conforming.

This matters for several reasons. Screen readers and other assistive technologies rely on the lang attribute—not xml:lang—to determine pronunciation and language-specific behavior. Search engines also use lang for content indexing and language detection. Having xml:lang without lang means the language declaration may be ignored entirely, degrading both accessibility and SEO.

In modern HTML5 documents, there is rarely a reason to include xml:lang at all. The lang attribute alone covers all use cases. The only scenario where you might need both is if your document must be compatible with both HTML and XHTML parsers (polyglot markup), in which case the two attributes must carry the same value.

How to Fix

You have two options:

1. Remove xml:lang and use only lang (recommended for HTML5 documents).
2. Add a lang attribute that matches the existing xml:lang value (for polyglot documents).

If you do keep both attributes, make sure the values are exactly the same—including case and subtags. For example, lang="en-US" must be paired with xml:lang="en-US", not xml:lang="en".

Examples

Incorrect: xml:lang without lang

<html xml:lang="en">

<head>

<title>My Page</title>

</head>

<body>

<p xml:lang="fr">Bonjour le monde</p>

</body>

</html>

This triggers the validation error because both the <html> and <p> elements have xml:lang but no lang attribute.

Incorrect: Mismatched values

<html lang="en" xml:lang="en-US">

<head>

<title>My Page</title>

</head>

<body>

<p>Hello world</p>

</body>

</html>

Even though both attributes are present, the values "en" and "en-US" don't match, which is also invalid.

Correct: Using only lang (recommended)

<!DOCTYPE html>

<html lang="en">

<head>

<title>My Page</title>

</head>

<body>

<p lang="fr">Bonjour le monde</p>

</body>

</html>

This is the cleanest approach for HTML5 documents. The lang attribute is all you need.

Correct: Both attributes with matching values (polyglot)

<!DOCTYPE html>

<html lang="en" xml:lang="en">

<head>

<title>My Page</title>

</head>

<body>

<p lang="fr" xml:lang="fr">Bonjour le monde</p>

</body>

</html>

If you must keep xml:lang, every element that has it must also have lang with the exact same value.