Attribute “xmlns:web” not allowed here.

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>

Learn more:

MDN Reference

MDN Web Docs: The html element

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WHATWG Spec

HTML Standard: Semantics, structure, and APIs of HTML documents

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MDN Reference

MDN Web Docs: HTML global attributes

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