The heart consists of 1,259 URLs from the lists. The :visited CSS pseudo-class is used here to highlight the sites visited before. Please read the DISCLAIMER below.
The list of domains are from the README of sites-using-cloudflare Github repository. Please visit it for detailed infomation.
Just because a site is highlighted above on the heart does not mean that it is compromised or you are affected by the cloudbleed. Sites may be compromised that do not appear and sites you have visited may not be highlight above either. The full list of urls composed the heart can be found here.
Quote from the source repository:
This list contains all domains that use Cloudflare DNS, not just the Cloudflare proxy (the affected service that leaked data). It's a broad sweeping list that includes everything. Just because a domain is on the list does not mean the site is compromised, and sites may be compromised that do not appear on this list.
Cloudflare has not provided an official list of affected domains, and likely will not due to privacy concerns. I'm compiling an unofficial list here so you know what sessions to reset and passwords to change.
See issue #127 and issue #87 for additional info about which sites are likely to be affected.