The encoding “utf8” is not the preferred name of the character encoding in use. The preferred name is “utf-8”. (Charmod C024)

The charset meta tag specifies an encoding name that is not the preferred form. Use utf-8 (with a hyphen) instead of utf8.

The HTML specification requires that character encoding declarations use the preferred IANA encoding name. For the Unicode UTF-8 encoding, the preferred name is utf-8, not utf8, UTF8, or other variations. While browsers may still recognize non-preferred names, the W3C validator flags them because the WHATWG HTML standard and IANA character set registry both list utf-8 as the canonical form.

This applies to the <meta charset> declaration and, less commonly, to charset parameters in Content-Type headers or <meta http-equiv> tags.

Incorrect example

<meta charset="utf8">

Correct example

<meta charset="utf-8">

Learn more:

MDN Reference

HTML meta element - MDN Web Docs

→

WHATWG Spec

Character encodings in HTML - WHATWG

→

web.dev Article

HTML document structure - web.dev

→