Mind. Blown.

I noticed this ages ago.

So, I'm a little confused. What's the HTT stand for? Also, from the thumbnail, all I could think of was, "NO U!".

It's actually "No, Thank You." (white letters) HTT is the CGI

Oh! and as for what HTT stood for I guess it's for "Houkago Tea Time"

In the second season ED No, Thank You, there's a short sequence where Ritsu, Yui, Mio, Azu-nyan and Mugi are arranged as stated from left to right.

Taking their initials, Tainaka, Hirasawa, Akiyama, Nakano, and Kotobuki, we form THANK.