Looking at the source it seems like it is a concatenation of a binary file (presumably the C64 binary) and some HTML, which hides the binary content through CSS

Looking at the source it seems like it is a concatenation of a binary file (presumably the C64 binary) and some HTML, which hides the binary content through CSS. The HTML is probably in the data part of the C64 binary. Interesting, but nowhere near as amazing as if you e.g. made a file that somehow both was a valid C64 binary and valid browser-interpretable (HTML/JS/whatever) file.