Bad value “csharp” for attribute “lang” on element “code”: The language subtag “csharp” is not a valid IANA language part of a language tag.

The lang attribute on any HTML element must be a valid IANA language subtag (like en, fr, or ja), not a programming language name.

The lang attribute specifies the natural (human) language of the element’s content, used by browsers, screen readers, and search engines for accessibility and localization purposes. Values like csharp, javascript, or python are not valid because they are not human languages registered in the IANA Language Subtag Registry.

This issue commonly arises when using <code lang="csharp"> to indicate the programming language of a code snippet. While the intention makes sense, lang is not the right attribute for this purpose.

To indicate a programming language on a <code> element, the recommended convention from the HTML specification is to use the class attribute with a language- prefix (e.g., class="language-csharp"). This is also the pattern used by popular syntax highlighting libraries like Prism.js and highlight.js.

If the code content is written in a specific human language (like English), you can still use lang="en" alongside the class.

Bad Example

<pre>
  <code lang="csharp">
    Console.WriteLine("Hello, World!");
  </code>
</pre>

Good Example

<pre>
  <code class="language-csharp">
    Console.WriteLine("Hello, World!");
  </code>
</pre>

Learn more:

MDN Reference

HTML lang attribute

→

www.iana.org

IANA Language Subtag Registry

→

MDN Reference

The code element

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