First of all, this tool does not test if your site is responsive. It only tests what happens when the window is resized. There's a difference. If you want to do proper responsive testing you should use something like BrowserStack which shows you the render results of your site on real devices.
The second and most major problem is that this tool uses IFRAMEs. What year is this? We all learned the lesson that IFRAMEs can be used for clickjacking more than 20 years ago and stopped using them as a result. In fact, the “Protect against clickjacking” feature in the .htaccess Maker prevents loading your site inside an IFRAME hosted on a different domain to prevent clickjacking. You can of course disable this feature, but I would only suggest doing that for a very short period of time.
Nicholas K. Dionysopoulos
Lead Developer and Director
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