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 A305701 #15 Jul 07 2018 01:48:17
%S A305701 0,10,100,101,102,110,120,201,210,1000,1001,1002,1010,1011,1012,1020,
%T A305701 1021,1022,1023,1032,1100,1101,1102,1110,1120,1200,1201,1202,1203,
%U A305701 1210,1220,1230,1302,1320,2001,2010,2011,2012,2013,2021,2031,2100,2101,2102,2103
%N A305701 Nonnegative integers whose decimal digits span an initial interval of {0,...,9}.
%H A305701 Robert Israel, <a href="/A305701/b305701.txt">Table of n, a(n) for n = 1..10000</a>
%p A305701 filter:= proc(n) local L;
%p A305701 L:= convert(convert(n,base,10),set);
%p A305701 L = {$0..max(L)}
%p A305701 end proc:
%p A305701 select(filter, [$0..3000]); # _Robert Israel_, Jun 10 2018
%t A305701 Select[Range[0,10000],Union[IntegerDigits[#]]==Range[0,Max[IntegerDigits[#]]]&]
%o A305701 (PARI) isok(n) = if (n==0, return (1)); my(d=Set(digits(n))); (vecmin(d) == 0) && (vecmax(d) == #d - 1); \\ _Michel Marcus_, Jul 05 2018
%Y A305701 Cf. A000670, A030299, A050289, A144688, A240763, A305712, A305714, A305715.
%K A305701 nonn,base
%O A305701 1,2
%A A305701 _Gus Wiseman_, Jun 08 2018