Content-Type is text/html; charset=utf-8
Agents sniff content type from the header before deciding how to parse a response. A page served with text/plain or no charset can be misinterpreted, especially when the body contains non-ASCII characters.
How the check decides
The check reads the response Content-Type header (lowercased) and asserts it contains both text/html and utf-8. Fails with the actual value (or (missing)) otherwise.
How to implement it
Configure your web server or CDN to serve .html files with Content-Type: text/html; charset=utf-8. Most platforms do this by default; explicit declarations matter when you’ve put the page behind a custom rewrite or worker.
Pass
HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
Fail
HTTP/1.1 200 OK
Content-Type: text/html