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 A047843 #22 Jan 08 2020 03:20:30
%S A047843 10,11,12,13,14,15,16,17,18,19,1011,21,1112,110213,11020314,
%T A047843 1102030415,110203040516,11020304050617,1102030405060718,
%U A047843 110203040506070819,100112,1112,22,1213,120314,12030415,1203040516
%N A047843 Describe n: give frequency of each digit, by increasing size; mention also missing digits between the smallest and largest one.
%C A047843 Other methods to describe or summarize n are: A047842 (as here, but ignoring "missing" digits), A244112 (count digits in order of decreasing size, ignoring missing digits). - _M. F. Hasler_, Feb 25 2018
%H A047843 Vincenzo Librandi, <a href="/A047843/b047843.txt">Table of n, a(n) for n = 0..5000</a>
%e A047843 131 contains two 1's, zero 2's and one 3, so a(131) = 210213.
%t A047843 a[n_] := Module[{T, f}, T = Tally[IntegerDigits[n]]; f[_] = 0; Do[f[t[[1]]] = t[[2]], {t, T}]; Table[{f[k], k}, {k, Min@T[[All, 1]], Max@T[[All, 1]]} ] // Flatten // FromDigits];
%t A047843 a /@ Range[0, 26] (* _Jean-François Alcover_, Jan 07 2020 *)
%o A047843 (PARI) A047843(n,S="")={if(n,for(d=vecmin(n=digits(n)),vecmax(n),S=Str(S,#select(t->t==d,n),d));eval(S),10)} \\ _M. F. Hasler_, Feb 25 2018
%Y A047843 Cf. A005151, A047841, A047842.
%K A047843 nonn,easy,base,nice
%O A047843 0,1
%A A047843 _N. J. A. Sloane_
%E A047843 More accurate title from _M. F. Hasler_, Feb 25 2018