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 A119098 #8 Jul 13 2021 01:24:31
%S A119098 1,2,6,21,66,81,206,253,666,681,4718,6666,6681,66666,66681,156341,
%T A119098 666666,666681,1497406,1569281,6666666,6666681,66666666,66666681,
%U A119098 666666666,666666681,6528600666,6666666666,6666666681,66666666666
%N A119098 Numbers k such that the k-th triangular number contains only digits {1,2,3}.
%H A119098 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr123.html">Tridigital Triangular Numbers</a>
%Y A119098 Cf. A000217, A119097. See A119034 for a table of cross-references.
%K A119098 nonn,base
%O A119098 1,2
%A A119098 _Giovanni Resta_, May 10 2006