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 A119217 #7 Jul 13 2021 21:30:57
%S A119217 12,132,947,1332,3867,13332,133332,299492,1222692,1333332,13333332,
%T A119217 133333332,1333333332,3944306652,13333333332,122020365307,
%U A119217 133333333332,394329425347,988724307827,1333333333332,12528986215052
%N A119217 Numbers k such that the k-th triangular number contains only digits {4,7,8}.
%H A119217 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr478.html">Tridigital Triangular Numbers</a>
%Y A119217 Cf. A000217, A119216. See A119034 for a table of cross-references.
%K A119217 nonn,base
%O A119217 1,1
%A A119217 _Giovanni Resta_, May 10 2006