Bad value “text/html; charset=iso-8859-15” for attribute “content” on element “meta”: “charset=” must be followed by “utf-8”.

The meta element's charset declaration must specify utf-8 when using HTML5. Values like iso-8859-15 are not valid in the HTML living standard.

In HTML5, the only permitted character encoding declared via a meta element is UTF-8. This applies to both the shorthand form (<meta charset="utf-8">) and the longer http-equiv form (<meta http-equiv="Content-Type" content="text/html; charset=utf-8">). Older encodings like iso-8859-15, windows-1252, or iso-8859-1 are not allowed.

The WHATWG HTML standard requires documents to be encoded in UTF-8. If a document uses a legacy encoding, the validator rejects it because the specification explicitly states that the charset value must be utf-8 (case-insensitive).

The shorthand <meta charset="utf-8"> is the preferred form in HTML5. It is shorter and does exactly the same thing as the http-equiv variant. There is no need to use both.

When switching from a legacy encoding, make sure the file itself is actually saved as UTF-8. Declaring UTF-8 in the markup while the file is saved in a different encoding will cause garbled characters. Most modern text editors let you choose the encoding when saving.

Examples

Invalid

<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15">

Valid

<meta charset="utf-8">

Learn more:

MDN Reference

HTML meta element - MDN

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MDN Reference

HTML Character Encodings - MDN

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WHATWG Spec

WHATWG HTML Standard - Semantics

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