HTML check: Attribute “isolang” not allowed on element “html” at this point.

About This HTML Issue

The W3C HTML Validator reports this error when it encounters isolang on the <html> element because isolang is not a recognized attribute in any version of HTML. This typically happens when developers attempt to specify the document’s language but use an incorrect or made-up attribute name, possibly confusing it with ISO language code terminology.

The correct attribute for declaring a document’s language is lang. This attribute accepts a valid BCP 47 language tag, which in most cases is a simple two-letter ISO 639-1 code (like en for English, fr for French, or pt for Portuguese). You can also use extended subtags for regional variants, such as en-US for American English or pt-BR for Brazilian Portuguese.

Setting the lang attribute properly is important for several reasons:

Accessibility: Screen readers use the lang attribute to select the correct pronunciation rules, ensuring content is read aloud accurately.
SEO: Search engines use the language declaration to serve the right content to users based on their language preferences.
Browser behavior: Browsers rely on lang for features like spell-checking, hyphenation, and selecting appropriate default fonts for the given language.
Standards compliance: Only recognized attributes pass W3C validation, and valid markup ensures consistent, predictable behavior across browsers.

To fix this issue, simply replace isolang with lang on your <html> element. Keep the same language code value—it’s the attribute name that’s wrong, not the value.

Examples

❌ Incorrect: Using the invalid `isolang` attribute

<!DOCTYPE html>
<html isolang="pt">
  <head>
    <title>Minha Página</title>
  </head>
  <body>
    <p>Olá, mundo!</p>
  </body>
</html>

This triggers the error: Attribute “isolang” not allowed on element “html” at this point.

✅ Correct: Using the `lang` attribute

<!DOCTYPE html>
<html lang="pt">
  <head>
    <title>Minha Página</title>
  </head>
  <body>
    <p>Olá, mundo!</p>
  </body>
</html>

✅ Correct: Using a regional language subtag

<!DOCTYPE html>
<html lang="pt-BR">
  <head>
    <title>Minha Página</title>
  </head>
  <body>
    <p>Olá, mundo!</p>
  </body>
</html>

Common language codes

Here are some frequently used ISO 639-1 language codes for the lang attribute:

en — English
es — Spanish
fr — French
de — German
pt — Portuguese
zh — Chinese
ja — Japanese
ar — Arabic
ko — Korean
ru — Russian

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Learn more:

en.wikipedia.org

Wikipedia: list of ISO 639-1 codes

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