I tried using <a href="view-source:google.com">External Source</a> but that just returns a broken link.
The schema/protocol http: is missing:
<a href="view-source:http:google.com">External Source</a>`
Test with the scURIple (scriple):
data:text/html;charset=utf-8,<html><a href="view-source:http:google.com">External Source</a></html>