Why is this an issue?

This rule raises an issue when an HTML element has a lang and/or xml:lang attribute, contains text content within its subtree, and the value of the attribute does not begin with a valid two‑letter ISO 639‑1 language code.

The lang attribute is used by assistive technologies, translation tools, and search engines to correctly interpret and process text content. If the value starts with an invalid language code — for example, a misspelled code or one not defined in ISO 639‑1 — these tools may mispronounce text, fail to translate it correctly, or misclassify the content’s language.

By ensuring that lang and xml:lang attributes start with a valid two‑letter language code, you improve accessibility, usability, and interoperability.

Noncompliant code examples

<html lang="engl">
  <head>
    <title>Example</title>
    <meta charset="utf-8" />
  </head>
  <body>
    <p>Welcome to our website</p> <!-- Noncompliant: "engl" does not start with a valid 2-letter code -->
  </body>
</html>

<html lang="en">
  <div lang="xx">
    Bienvenido a nuestro sitio web <!-- Noncompliant: "xx" is not a valid ISO 639-1 code -->
  </div>
</html>

Compliant solutions

<html lang="en">
  <head>
    <title>Example</title>
    <meta charset="utf-8" />
  </head>
  <body>
    <p>Welcome to our website</p> <!-- Valid: "en" is a valid ISO 639-1 code -->
  </body>
</html>

<html lang="en">
  <div lang="es">
    Bienvenido a nuestro sitio web <!-- Valid: "es" is a valid ISO 639-1 code -->
  </div>
</html>

Resources