NeoCities rekindled my interest in 88x31 website buttons and I began to wonder about the size standard, which has persisted for two decades. Why 88 by 31, anyway? Who started this? A Quora poster speculates GeoCities started this trend when they provided 88x31 GeoCities buttons for their users, but the contemporary source she references actually says the trend was started by Netscape.
Netscape, for those of you who have been living in a bubble or don't have graphical web access, started it all, with their "Netscape Now!" buttons. Other people started making their own buttons, using the same style but substituting their own graphics and text. (Many of them kept the "Now!" lettering, as that's part of the joke.) And there are some who have adopted the dimensions but have completely reinterpeted the button.
The Internet Archive appears to support this. The earliest Netscape site archived, captured on 10/20/96, features a Netscape Now! 3.0 button and additional navigation buttons that are also beveled 88x31 buttons with a gray background. The earliest GeoCitites site archived two days later on 10/22/96 doesn't feature a GeoCities button but has several other 88x31 buttons in the sidebar. The FAQ requests that users link their free pages back to the main page, but no buttons are provided or referenced.