Use HTML and CSS for content monitoring check

Hi, I want to use the monitoring for websites and I in the page I put this line of HTML and CSS <p style="display: none;">OK_UP</p> son when the service looks for that string and for example the page has been modified  and don't show that string the is going to mark it as down, I set up pingdom and tested the service and it detects if I change that string, I tried to do the same with anturis but it is not seeing that text,
I tried in "HEADER found in" OK_UP and "BODY found in" OK_UP but got the same result.

Thanks in advance.