Object enumeration order (before ES2020 [1]), math function accuracy (Firefox and Chrome did standardize on fdlibm though [2]), various timings. All of them can be fixed with caution but it might be easier to use a platform where none of them matters (well, as long as you don't import Math.sin etc. from the environment...).
In many languages like those used to compile to WebAssembly (c++, c#, rust) enumerating standard dictionaries is non-deterministic. A javascript object is a dictionary, it is not surprising that it had an undefined order. If you need order use an array, if you need to lookup a value quickly use a dictionary, if you need both then you compose some ordered dictionary structure that internally has a dictionary and an array.
I guess since so many js devs never understood this and built code around one js engines property order behavior they had to go and specify it as a standard and now all js engine must add extra overhead to track and maintain property order...
Not sure how webassembly helps object enumeration order, it doesn't have objects to enumerate! To make objects you compile to webassembly from languages that most likely have undefined dictionary order and may not even have reflection to enumerate an object at runtime.
Look how much spread there is in the JS, and how little there is in the wasm.
Depending on what you're doing, this may or may not matter to you.