[lhorie]

JS has two top level grammars, one defaults to loose mode and the other defaults to strict mode, among other nuances.

Possibly the most devious nuance is whether the spec's appendix B applies, which affects whether html comment syntax is valid (yes, this is a thing). The html comment token can therefore be parsed either as a comment or as a series of operators depending on the grammar being used.

Effectively, this means it's possible to craft a program that does different things in CJS vs ESM mode.