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 A119200 #12 Mar 27 2023 07:36:18
%S A119200 3,78,378,8778,887778,88877778,8787733878,8888777778,73778888778,
%T A119200 888887777778,8883333788878,37383337333378,88888877777778,
%U A119200 8888888777777778,888888887777777778,88888888877777777778,8888888888777777777778,887783783833373788333378
%N A119200 Triangular numbers composed of digits {3,7,8}.
%H A119200 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr378.html">Tridigital Triangular Numbers</a>.
%F A119200 a(n) = A000217(A119201(n)). - _Tyler Busby_, Mar 24 2023
%Y A119200 Cf. A000217, A119201. See A119033 for a table of cross-references.
%K A119200 nonn,base
%O A119200 1,1
%A A119200 _Giovanni Resta_, May 10 2006
%E A119200 a(17)-a(18) from _Tyler Busby_, Mar 24 2023