Document compatibility mode not set
This means that the URL in question does not have the document compatibility mode set in the <head> or in the HTTP headers.
Why is this important?
The document compatibility mode can be set in the <head> and in HTTP headers. Internet Explorer 8/9/10/11 support document compatibility modes. Setting the document compatibility allows you to control what rendering engine is used, rather than Internet Explorer deciding for itself. For example, without this setting, it is possible for Internet Explorer 9 to use the Internet Explorer 6 rendering engine.
What does the Hint check?
This Hint will trigger for any internal HTML URL which does not set the document compatibility in the <head> or in the HTTP header.
Why is this Hint marked 'Insight'?
This Hint is an 'Insight', which means there isn't necessarily any action that needs to be taken - the Hint is intended to alert your attention to something, rather than flagging up an issue that needs fixing. In this instance, the Hint relates only to older versions of Internet Explorer, the website may receive very little traffic from such browsers.
It is recommended that you always try to make sure your web pages work correctly in the latest versions of IE. There are times, however, when you may need to temporarily tell Internet Explorer to display a page or website in a document mode used by a previous version of the browser which makes being able to specify the document compatibility modes a handy feature to have.
For a small web site simply dropping in the X-UA-Compatible meta tag is not a large task but if you have a web site with tens of thousands of pages, adding and maintaining the meta tags could be quite a large task. The better approach if you need to include the X-UA-Compatible value for an entire web site is to include the value as an HTTP Response Header.