Looks like it might have been VisualLand.net, which isn't around anymore. Looking at the archive of the site (https://web.archive.org/web/20130811073141/http%3A//www.visu...), most of their animations / diagrams were Flash, so no longer run, and don't seem to work with archive.org's emulator either.
No, that's not it unfortunately. It was a single page with a style of diagrams that shows a high-level format of something, with arrows to a zoomed-in version that details the structure of the fields. I remember its colour scheme being dominated by light blue and green, IIRC.
Could this be it? TLS 1.3 connection, with every field in the packets labelled: https://tls13.xargs.org
No, not that one either. The page was static and full of high quality images dissecting the format structures.