I had fun re-exploring some games that I played decades ago :)

From the wikipedia page: https://en.wikipedia.org/wiki/Noclip.website

It therefore allows exploration in ways not intended by the game's developers, providing new insights into their layout, development process, and content. The site was programmed by JasperRLZ and a team of contributors, and predominantly features Nintendo games, although it also contains other titles, such as Dark Souls.[1] The site was praised by critics for allowing visitors to explore levels from classic games, and its implementation of bespoke solutions allowing maps from each game to run properly within the same viewer.

noclip.website is open-source software. When visiting the site, a list of levels is presented, and the visitor may select one level at a time to view, navigating it using WASD and mouse controls similar to those of an ingame noclip mode

The creator has a channel here, with explainers on various game design / rendering: https://www.youtube.com/@JasperRLZ