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.
?(Mod[#,3]==0&)]]-1 (* or *) LinearRecurrence[{1,0,1,-1},{0,4,8,9},100] (* _Harvey P. Dale has authored 0 sequences.