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After a long way of troubleshoot on this issue, it seems like the main issue is not there is extra <p> tag in the html, but :
That's because there have nested blocks element inside the p tag which is invalid. Inside p tag, it only can nest inline elements such as span, a and img. So the markup is invalid
And seems like the invalid markup is come from Jomsocial :( in the file:
/public_html/components/com_community emplates/jomsocial/layouts/stream/base-extended.php
Line 84
www.screencast.com/t/DLqyBs14qs
I've remove the code and clear the cache
Seems the w3 validator
validator.w3.org/nu/?doc=http%3A%2F%2Fwww.hl7community.org%2F
did not shows the error anymore.
Perhaps, can you consult with Jomsocial, is it the data-type="stream-content" do any ajax call, because as I look on the layout it doesn't effected.