Technically, even semver doesn't strictly require you to not update the major version when there haven't been incompatible changes (I swear, a triple negative is the easiest way to parse this sentence :P ).
The web tries very hard to avoid backwards incompatibilities. I don't expect the WASM standard to ever directly get any backwards incompatibilities, unless they're opt-in (like how using JS modules stops you from using with-statements).
