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.
| n | a(n) |
|---|---|
| 1 | 990 |
| 2 | 4009490400090 |
| 3 | 990440044904409490 |
| 4 | 9900404040400404004090 |