Bad value “yaml” for attribute “lang” on element “code”: Found reserved language tag: “yaml”.

The language tag yaml is reserved by IANA and cannot be used as a value for the lang attribute, which expects a valid BCP 47 language tag (like en for English or fr for French).

The lang attribute specifies the natural language of an element’s content — human languages like English, Spanish, or Japanese. It is not meant to indicate a programming or markup language. When you write lang="yaml" on a <code> element, the validator rejects it because yaml is a reserved IANA subtag with no valid use in BCP 47.

If your goal is to identify the code language for syntax highlighting or styling purposes, use the class attribute instead. A common convention, recommended by the HTML specification itself, is to use a class prefixed with language-, such as class="language-yaml".

HTML Examples

❌ Invalid: using lang for code language

<pre>
  <code lang="yaml">
    name: my-project
    version: 1.0.0
  </code>
</pre>

✅ Valid: using class for code language

<pre>
  <code class="language-yaml">
    name: my-project
    version: 1.0.0
  </code>
</pre>

This class="language-*" convention is widely supported by syntax highlighting libraries like Prism.js and highlight.js.

Learn more:

MDN Reference

HTML lang attribute

→

MDN Reference

The code element

→

MDN Reference

BCP 47 language tags

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