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 A004687 #21 Sep 08 2022 08:44:33
%S A004687 0,1,1,2,3,11,20,31,111,202,313,1121,2100,3221,11321,21202,33123,
%T A004687 120331,220120,1001111,1221231,2223002,10110233,12333301,23110200,
%U A004687 102110101,131220301,233331002,1031211303,1331202311
%N A004687 Fibonacci numbers written in base 4.
%H A004687 Vincenzo Librandi, <a href="/A004687/b004687.txt">Table of n, a(n) for n = 0..1000</a>
%t A004687 bf[n_,k_]:=Module[{s=ToString[BaseForm[n,k]]},ToExpression[StringDrop[s,{Part[StringPosition[s,"\n"],1,1],-1}]]] lst={};Do[f=bf[Fibonacci[n],4];AppendTo[lst,f],{n,0,4*4!}];lst (* _Vladimir Joseph Stephan Orlovsky_, Jun 17 2009 *)
%t A004687 FromDigits[IntegerDigits[#,4]]&/@Fibonacci[Range[0,30]] (* _Harvey P. Dale_, Jun 24 2011 *)
%o A004687 (PARI) vector(50, n, n--; fromdigits(digits(fibonacci(n), 4))) \\ _G. C. Greubel_, Oct 09 2018
%o A004687 (Magma) [Seqint(Intseq(Fibonacci(n),4)): n in [0..50]]; // _G. C. Greubel_, Oct 09 2018
%Y A004687 Cf. A000045 (Fibonacci), A007090 (numbers in base 4).
%K A004687 nonn,base,easy
%O A004687 0,4
%A A004687 _N. J. A. Sloane_