HTML Guides for ISO 639-1
Learn how to identify and fix common HTML validation errors flagged by the W3C Validator — so your pages are standards-compliant and render correctly across every browser. Also check our Accessibility Guides.
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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