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 A119201 #7 Jul 13 2021 21:29:38
%S A119201 2,12,27,132,1332,13332,132572,133332,384132,1333332,4215052,8646772,
%T A119201 13333332,133333332,1333333332,13333333332,133333333332,1332504246772,
%U A119201 1333333333332,2734350887427,13329578968507,13333333333332
%N A119201 Numbers k such that the k-th triangular number contains only digits {3,7,8}.
%H A119201 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr378.html">Tridigital Triangular Numbers</a>
%Y A119201 Cf. A000217, A119200. See A119034 for a table of cross-references.
%K A119201 nonn,base
%O A119201 1,1
%A A119201 _Giovanni Resta_, May 10 2006