HTML Guide - issues tagged as xmlns

The “xmlns:o” attribute is not allowed because it is not part of the standard HTML attributes and typically appears due to improperly embedded XML or Office-related HTML content.

In HTML documents, the “xmlns” attribute is used to declare a namespace and is generally valid for certain elements when dealing with XML documents or XHTML. However, the “xmlns:o” attribute specifically relates to XML namespaces typically used in Microsoft’s Office XML formats (for example, when copying and pasting styled content from Word to an HTML editor). Since HTML5 doesn’t natively support these namespaces as valid HTML attributes and strictly validates against them, any non-standard attributes like “xmlns:o” generate validation errors.

To fix this issue, you should remove the “xmlns:o” attribute unless you are sure that it’s necessary for the function of your document, and the document should be processed in a way that supports such namespaces. If the content is not meant to include Office-specific data elements, it’s likely this attribute was accidentally included and can be safely removed.

Example of Incorrect HTML with “xmlns:o”

<!DOCTYPE html>
<html>
<head>
  <title>Sample Document</title>
</head>
<body xmlns:o="urn:schemas-microsoft-com:office:office">
  <p>This is a paragraph within a body tag falsely including an xmlns:o attribute.</p>
</body>
</html>

Corrected Example without “xmlns:o”

<!DOCTYPE html>
<html>
<head>
  <title>Sample Document</title>
</head>
<body>
  <p>This is a paragraph within a correctly structured HTML document.</p>
</body>
</html>

By removing the “xmlns:o” attribute, the HTML document complies with the W3C standards, leading to successful validation. If you require namespaces for XML processing, it’s essential to handle them outside the context of standard HTML or within an XML or XHTML document structure where such namespaces are appropriate and valid.

Attribute “xmlns:serif” not allowed here.

Bad value “” for the attribute “xmlns” (only “http://www.w3.org/2000/svg” permitted here).

Ensure the xmlns attribute for <svg> elements is set to “http://www.w3.org/2000/svg”.

In HTML documents, the xmlns attribute in an <svg> element must be defined correctly. The xmlns attribute specifies the XML namespace for an SVG element, which should always be “http://www.w3.org/2000/svg”. If you receive a validation error indicating a bad value for xmlns, it means that the attribute is empty or incorrectly set. This namespace ensures that the SVG elements are properly recognized as SVGs by the browser and aids in maintaining proper structure and function.

Here’s a correct example of an <svg> element:

<!DOCTYPE html>
<html lang="en">
  <body>
    <svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
      <circle cx="50" cy="50" r="40" stroke="black" stroke-width="3" fill="red" />
    </svg>
  </body>
</html>

Ensure that every <svg> element you use includes the xmlns attribute with the correct value to avoid validation issues and ensure complete browser compatibility.

Bad value “https://www.w3.org/1999/xlink” for the attribute “xmlns:link” (only “http://www.w3.org/1999/xlink” permitted here).

Bad value “https://www.w3.org/2000/svg” for the attribute “xmlns” (only “http://www.w3.org/2000/svg” permitted here).

The only permitted value for the xmlns attribute is http://www.w3.org/2000/svg.

The namespace declaration for an <svg> element is provided by the xmlns attribute like this:

<svg xmlns="http://www.w3.org/2000/svg">
  <!-- more tags here -->
</svg>

Although using https in the URL looks like it’s going to be more secure, in fact this URL is not used to connect it to, only to declare the namespace.