Confirms that the webpage is declared to be an HTML page with UTF8 encoding.
The "Content-Type" header declares what type of content is being displayed. In the case of a webpage, this should be "text/html".
Additionally, this header allows you to specify the character encoding of the content. If the character set is omitted, the default encoding is going to be "ISO-8859-1" which may not be what you want. The recommended value for this is UTF-8 since this is a common encoding that supports any multibyte character.
These two values are combined with a semi colon. The ideal value for a webpage is therefore "text/html; charset=utf-8". Any deviation from this will cause this test to issue a warning.
For more information, please read the documentation on MDN Web Docs.