Could be, one of possible ways for Website which is using a Cloudflare service(s) for protection measures.
Cite:
…It also challenges visitors without a user agent or with a non-standard user agent such as commonly used by abusive bots, crawlers, or visitors…
As we know, Cloudflare offers these services to websites that want to protect their servers from Denial-Of-Service (DDoS) and other kinds of attacks.
There are 2 potential reasons you encountered this at your or someone else’s Website:
- Under Firewall > Settings you have activated ‘I’m under attack!’.
- Under Firewall > Rules you have set up a firewall rule to perform a JavaScript challenge.
If this is your Website and you cannot access it, you should diagnose what is going on and you would have to find which one is causing the challenge and disable it or modify it to prevent future events like this.
Moreover, mostly if you encounter the Checking your browser message, it simply means that the website you’re trying to access uses Cloudflare to protect their page against DDoS attacks. For this reason, a temporary page will be presented to you containing the message mentioned above, and at the same time, the servers are checking if your PC is real.
If this happens to you daily, related to the above points 1. or 2., it could also be due to some Web browser extension, or if using VPN and the IP address is suspicious to Cloudflare, or maybe something with privacy and cookies in our Web browser.
I hope I understood it good as far as I use it for the past few years and also provide you some helpful information regarding your question 
What if the check fails? What is displayed then?
I believe, if it checked and all good, then it should pass to the requested resource/webpage.
Moreover, if it is a captcha, then if you as a human cannot successfully complete the captcha challenge it goes again once more - possible to be in a “loop” until you successfully complete it.
Otherwise, should show an CF error like “1010 The owner of this website has banned your access based on your browser’s signature” (browser integrity check), “1006 Access Denied: Your IP address has been banned”, “Access denied (Error: 1020)”, etc. or some similar due to the Firewall rules and other security options being enabled.
https://support.cloudflare.com/hc/en-us/articles/360029779472-Troubleshooting-Cloudflare-1XXX-errors
What if I repeatedly “refresh” a page, will that trigger rate limiting?
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