Unmappable byte sequence: X.

The document was decoded with a legacy encoding, usually windows-1252, and contains a byte that has no character in that encoding.

A character encoding maps bytes to characters. In windows-1252, five byte values map to nothing: 81, 8d, 8f, 90 and 9d. The message gives the byte that failed in hexadecimal.

Most of the time the file is saved as UTF-8, but nothing says so. With no <meta charset> in the page and no charset in the HTTP Content-Type header, the validator has to guess the encoding and often picks windows-1252. UTF-8 stores accented letters as two bytes, and for some of them the second byte is one of the unmapped values: Í is c3 8d and č is c4 8d. Read as windows-1252, the first byte turns into the wrong character (Ã or Ä) and the second can't be decoded at all. A browser that guesses the same way shows that garbled text to visitors.

Declare UTF-8 with <meta charset="utf-8"> as the first element inside <head>. This also clears the "The character encoding was not declared" error that usually comes with this one. If the server sends a Content-Type header with a legacy charset such as iso-8859-1, change the server configuration as well, because the header takes priority over the <meta> element.

If the file really is saved as windows-1252, the byte is broken data, often from text pasted out of a document in another encoding. Convert the file to UTF-8, then find the damaged character and type it again.

Invalid example

A file saved as UTF-8 and served without a charset:

<!DOCTYPE html>
<html lang="es">
  <head>
    <title>Índice</title>
  </head>
  <body>
    <h1>Índice</h1>
  </body>
</html>

Valid example

<!DOCTYPE html>
<html lang="es">
  <head>
    <meta charset="utf-8">
    <title>Índice</title>
  </head>
  <body>
    <h1>Índice</h1>
  </body>
</html>

Learn more:

MDN Reference

MDN Web Docs: The meta element

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MDN Reference

MDN Web Docs: Character encoding

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WHATWG Spec

HTML Standard: Parsing HTML documents

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