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.
We are taking care of free subdomain users. That’s why we have GoGetSSL and ZeroSSL providers (which do not have the same certificate limit). ZeroSSL especially is just as easy to use as Let’s Encrypt.
Having an SSL certificate doesn’t make a site trustworthy. That’s not what an SSL certificate does, or has ever done, despite the marketing nonsense to the contrary.
The only things an SSL certificate ensures is that you are connected to the website that’s in the address bar and the data transmitted cannot be seen or manipulated by a third party.
So if you send credit card details to a website, an SSL certificate ensures that nobody else but you and the website can see the credit card details. It provides absolutely zero guarantee that the website will actually provide the services or products you paid for, or that they won’t sell your card details on the dark web.