Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
The src read-only property of the NotRestoredReasons interface returns a string representing the path to the source of the <iframe> the document is contained in (for example <iframe src="b.html">).
A string.
If the document is not in an <iframe>, src will return null.
See Monitoring bfcache blocking reasons for examples.