I was online during that era but "88x31" does not ring any bells. Is it significant at all or just some arbitrary size? What about the number of animation frames? 88x31x10?
Then it became an odd ad-hoc standard of that era of the internet, where you'd include 88x31 buttons because it was cool and popular. While others would be advertising their page based on how cool the buttons looked, and making them fit that standard 88x31 buttons so they were more likely to be included.
Edit: Oh, someone answered in another thread - appears to be as another person remembered here: https://neonaut.neocities.org/cyber/88x31.html