Add information about which providers already have existing CA validation

Under the "Existing CA validation" section of the Certification page, there is this sentence, "Some existing commercial providers already provide this service." -- could you provide, perhaps as a footnote, or reference, links to the providers that you know are already providing this service?

In the context of pitching this for funding, if I was reading this and I came to this section and I found that there were already CAs that provide this service, then I would wonder why I would want to fund this if it already exists, so perhaps a sentence or two about the pros/cons of the existing implementations, or why doing it another way would be beneficial would be helpful!