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 A319953 #19 Feb 20 2021 17:23:08
%S A319953 2,20,21,200,201,210,211,2000,2001,2010,2011,2100,2101,2110,2111,
%T A319953 20000,20001,20010,20011,20100,20101,20110,20111,21000,21001,21010,
%U A319953 21011,21100,21101,21110,21111,200000,200001,200010,200011,200100,200101,200110,200111
%N A319953 List of binary words of lengths 0, 1, 2, etc., including empty word, each prefixed by a 2.
%H A319953 Rémy Sigrist, <a href="/A319953/b319953.txt">Table of n, a(n) for n = 0..25000</a>
%H A319953 Carl Pomerance, John Michael Robson, and Jeffrey Shallit, <a href="https://doi.org/10.1016/S0304-3975(96)00189-2">Automaticity II: Descriptional complexity in the unary case</a>, Theoretical computer science 180.1-2 (1997): 181-201.
%F A319953 a(n) = A007088(n+1) + 10^A000523(n+1). - _Rémy Sigrist_, Oct 04 2018
%o A319953 (PARI) a(n) = my (b=binary(n+1)); b[1]++; fromdigits(b) \\ _Rémy Sigrist_, Oct 04 2018
%o A319953 (Python)
%o A319953 def a(n): return int(bin(n)[2:].replace('1', '2', 1))
%o A319953 print([a(n) for n in range(1, 40)]) # _Michael S. Branicky_, Feb 20 2021
%Y A319953 Cf. A000523, A007088, A214339, A319954.
%K A319953 nonn,base
%O A319953 0,1
%A A319953 _N. J. A. Sloane_, Oct 04 2018
%E A319953 Data corrected and extended by _Rémy Sigrist_, Oct 04 2018