This question is an exact duplicate of:
Stack Exchange sites use CloudFlare. When an IP address is used quite a lot to access Stack Exchange sites, CloudFlare will present a CAPTCHA to be solved. After it's solved, the site is supposed to be normally usable.
The problem is that solving the CAPTCHA for a Stack Exchange site doesn't remove the need to solve a separate CAPTCHA for Stack Exchange's CDN servers. This leads to people solving the CAPTCHA for the site they're trying to access and receiving pages with no CSS or images. This is something a lot of users aren't going to know how to correct.