The first time I tried to embed this map from ArcGIS Online at UMass Amherst, using the simple method of pasting the shared URL on it’s own line, I got a blank page. Why?

Troubleshooting efforts:
Q. Is the site on safe list supported by WordPress native oEmbed, or the iFramely plugin?
A. Usually a page that’s not supported simply shows a link instead of a blank, as in this example of an Adobe SPARK page:

Adobe Spark | Gallery > Photography > Twelve in Twelve-2016

Q. Is the site and all it’s content (including images) served up securely via HTTPS?
Look at the URL on this page, if it’s not you will see an alert icon instead of a green lock. Also check the source page.
A. The page I’m trying to embed is secure. Check it out, viewed on its own page.

Q. Does the service hosting the app have sharing or privacy settings?
A.  Yes! That was it.  I had neglected to set the sharing for the map and app to “public.”

ArcGIS Web Application