The reason Let’s Encrypt doesn’t work for subdomains is that Let’s Encrypt has limits on how many certificates they will issue for a certain registered domain name. That limit is 50 certificates per week right now.
So that means that Let’s Encrypt will only provide 50 certificates per week for html-5.me and ALL subdomains. And if you look at how many certificates we issue for, say, epizy.com , 50 per week is nowhere near enough to provide a certificate to everyone.
So why could you get one for html-5.me ? Because I forgot to add it to the list of domains for subdomains. But that’s fixed now.