There are a varierty of websites using CACert, unfortunately all get "F" simply because CACert is not a recognized issuer.
For more details: http://www.cacert.org/
Because SSL Labs relies on Mozilla's root store, I am afraid that there isn't anything we can do about that until Mozilla accepts CAcert as trusted. We soon plan to introduce multiple stores to the test, so if CAcert is other stores, we will indicate their certificates as trusted (for those other stores).
It's not supported by most browsers yet.
I tested IE11 and a recent Firefox and confirmed they didn't trust the certs they produce. Given this is the case, and a user is forced to assume their own level of trust, that's exactly the definition of an broken chain of trust. Accordingly, it's a broken view of a secure website. SSL Labs do the right thing by issuing an 'F' grade.
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