I have examined together with @patomation an analysis of a webpack bundle that includes @cycle/dom and we have found in it that both CommonJS and ES snabbdom modules are included.

From my understanding of imports and webpack and snabbdom, here's what may be wrong: the above line is the only import in @cycle/dom code that is resolved to an ES snabbdom module, possibly because webpack may be reading this:

https://github.com/snabbdom/snabbdom/blob/665d9d39ff874955254d89b381c392a6ae4e94b1/package.json#L9

And all of the rest of the snabbdom imports refer directly to CommonJS module paths inside of the package. For example:

So we end up with some duplicate snabbdom modules in the bundle and even worse — most of them are CommonJS.

This while snabbdom does provide an ES modules build.