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.
The top left 8 X 8 corner of the array shows how this surjective self-avoiding walk begins (connect the terms in numerical order, 0-1-2-3-...): 0 1 14 15 16 19 20 21 3 2 13 12 17 18 23 22 4 7 8 11 30 29 24 25 5 6 9 10 31 28 27 26 58 57 54 53 32 35 36 37 59 56 55 52 33 34 39 38 60 61 50 51 46 45 40 41 63 62 49 48 47 44 43 42
b[{n_, k_}, {m_}] := (A[k, n] = m-1);
MapIndexed[b, List @@ HilbertCurve[4][[1]]];
Table[A[n-k, k], {n, 0, 12}, {k, n, 0, -1}] // Flatten (* Jean-François Alcover, Mar 07 2021 *)
The top left 8x8 corner of this array: +0 +1 +7 +5 16 17 20 21 +3 +2 +6 +4 18 19 23 22 +9 +8 14 13 28 29 27 25 10 11 12 15 31 30 24 26 48 51 52 53 47 46 39 37 50 49 55 54 45 44 36 38 60 61 57 59 35 34 40 41 63 62 56 58 32 33 43 42