[icebraining]

LE is tied to a root CA (IdenTrust's). The support is almost universal, with only obsolete OSs not trusting them: https://community.letsencrypt.org/t/which-browsers-and-opera...

[DisposableMike]

Despite a fairly large number of users on XP still (2.5% of total users on some sites I manage), I'll give you that it works on non-obsolete OS browsers. However, those are not the only pieces in the world of security.

Java, for example, only started support as recently as 3 weeks ago (2016-07-19)

[pfg]

XP is supported. There was an issue due to some schannel bug in XP choking on the issuer certificate, but that was fixed earlier this year.

[true_religion]

Lot's of people care about and make their money off users with "obsolete OS's and browsers".

[icebraining]

And they're just as likely to have problems with any other CA.

[darklajid]

Including HN / ycombinator?