Why does Microsoft Edge (formerly Project Spartan) prompt to open this website in Internet Explorer?

Question

Does anyone know what triggers MS Edge to not open a specific web page/site, but instead prompt the user to open the site in internet explorer?

The message says:

Answer 1

There is no rule set based on the content of the page to cause this prompt.

The prompt is controlled using the "Compatibility List" which is a cloud-hosted list downloaded by the MS Edge browser periodically. It is very similar to the CV list in IE.

In most cases we are working with the sites on that list to migrate to newer technologies at which time they'll be removed from the list.