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 A080483 #6 Jun 10 2019 13:38:29
%S A080483 0,21,543,9876,1413121110,201918171615,27262524232221,
%T A080483 3534333231302928,444342414039383736,54535251504948474645,
%U A080483 6564636261605958575655,777675747372717069686766,90898887868584838281807978
%N A080483 Reverse concatenation of next n numbers with a(1) = 0.
%C A080483 a(n) is a multiple of 3.
%H A080483 Harvey P. Dale, <a href="/A080483/b080483.txt">Table of n, a(n) for n = 1..200</a>
%t A080483 With[{nn=20},FromDigits[Flatten[IntegerDigits[#]]]&/@(Reverse/@ TakeList[ Range[0,(nn(nn+1))/2],Range[nn]])] (* Requires Mathematica version 11 or later *) (* _Harvey P. Dale_, Jun 10 2019 *)
%Y A080483 Cf. A080481, A080482, A080484.
%K A080483 base,easy,nonn
%O A080483 1,2
%A A080483 _Amarnath Murthy_, Mar 11 2003
%E A080483 More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Apr 06 2003