Document compatibility mode in <head> and headers differ
This means that the URL in question has the document compatibility mode set in the <head> and the HTTP headers, and their values are different.
Why is this important?
The document compatibility mode can be set in the <head> and in HTTP headers. If it is set in both places, yet their values are different, this could lead to the incorrect mode being used, because the <head> value will be ignored as the HTTP Header takes priority.
What does the Hint check?
This Hint will trigger for any internal HTML URL which has different values set for document compatibility in the <head> and the HTTP header.
Examples that trigger this Hint:
The Hint would trigger for this URL if it had both of the following;
Document compatibility set in the <head>:
AND set in the HTTP Header, but to a different value:
How do you resolve this issue?
Either ensure that the two values match, or change the setup so that the document compatibility mode is only set in one location.