About This HTML Issue
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">
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