This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A089431 #5 Aug 20 2017 23:16:39 %S A089431 1,2,12,120,840,5040,55440,720720,720720,24504480,465585120 %N A089431 Even-indexed terms of A089423. %K A089431 nonn %O A089431 0,2 %A A089431 _Antti Karttunen_, Nov 29 2003