I am trying to see how well Let’s encrpt 's certificate are trusted by different browsers; when I did some google, I found https://helloworld.letsencrypt.org/
Unfortunately this site is showing as untrusted on Chrome/Firefox/IE.
It’s signed with a “Let’s Encrypt Authority X1” . If this site is a demo for Letsencrypt, please fix its certificate issue on it.
It has to be like that to meet the requirements of a test site for root inclusion in Mozilla.
It could have been any arbitrary site, wouldn't it? Why helloworld was chosen, I dunno.. 
Because the site of the 1st certificate is the natural choice.
Mmm. This certificate expires in less than two weeks. Of course it’s entirely possible that Kathleen will decide some time in that next two weeks to complete the review period for ISRG, but if she doesn’t hopefully Let’s Encrypt have a plan for how the site gets a certificate that is still signed by ISRG, yet also still valid. It’s good to have such a plan now, rather than figuring it out only when an awkward question is asked on m.d.s.policy or in Bugzilla