CloudFlare provides a reverse proxy, and it offers SSL support ("flexible", "full", "strict full", and "keyless").

How does CloudFlare manage to get a valid certificate for domains it does not own? Don't the certificate authorities [normally require proof](https://security.stackexchange.com/q/53408/15712) that you own the domain, at the very least sending a verification email to an email address of that domain? How does CloudFlare skip this step?