Cross-browser X Library
I can’t recall when I first checked out the X library cross-browser.com but it must have been back in the prototype/scriptaculous days.
The last release of the library was X 4.23 from 14-May-2011, and I suspect was subsequently eclipsed by the rise of the jQuery ecosystem. The X library has some really advanced capabilities that it is a pity never went any further, but the coding paradigm of the X library was perhaps just not as coherent as the jQuery approach.
About the X library:
Inner Solar System Demo
The inner_solar_system.html demo remains one of the most popular, despite using an earlier version of the X library. As of 2020, it still runs jsut fine in the latest Chrome and Firefox browsers.