@fdwr raised this issue when reviewing Chromium CL https://chromium-review.googlesource.com/c/chromium/src/+/6972256/comment/7552dd15_61137e37/

The WebNN spec says in the table that ranks 0 to N are allowed for pad's input, including scalars which is mathematically reasonable (given beginningPadding and endingPadding are both empty to satisfy the rule that they have the same size as the input rank) and also consistent with tile and transpose as nops, and you even have an implementation of it below. Though, currently one of the algorithm steps say "If input’s rank is 0, then throw a TypeError", and so I'd be happy to update the spec to remove that 1 line in the algorithm steps.