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 A001369 #24 Feb 01 2022 01:34:12
%S A001369 1,23,456,7891,1234,567891,123456,78910123,456789101,2345678910,
%T A001369 12345678910,123456789101,2345678910123,45678910123456,
%U A001369 789101234567891,123456789101234,56789101234567891,12345678910123456,7891012345678910123,45678910123456789101
%N A001369 Blocks of increasing length using 1,2,3,...,9,10; omit leading 0's.
%C A001369 It appears that the first digit repeats 1, 2, 4, 7, 1, 5, 1, 7, 4, 2, 1. - _T. D. Noe_, Apr 05 2011
%H A001369 Vincenzo Librandi, <a href="/A001369/b001369.txt">Table of n, a(n) for n = 1..200</a>
%t A001369 nn = 20; d = Flatten[Table[{1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 0}, {Ceiling[nn (nn + 1)/22]}]]; Table[e = (n + 1) n/2; s = e - n + 1; FromDigits[d[[s ;; e]]], {n, nn}] (* _T. D. Noe_, Apr 05 2011 *)
%o A001369 (PARI) N=[]; k=Mod(-1,10); for(n=1, 20, while(#N<n, N=concat(N, Vec(Str(1+lift(k++))))); print1(eval(concat(vecextract(N, 2^n-1)))", "); N=vecextract(N, Str("^.."n))) \\ _M. F. Hasler_, May 08 2014
%Y A001369 Cf. A066547, A007923.
%K A001369 nonn,base
%O A001369 1,2
%A A001369 _N. J. A. Sloane_