Yes, there a to many unknowns when dealing with a polyfill and experimental JS. IntersectionObserver will be a thing the moment Apple embraces it … and with some luck people update quickly. At least on iOS adoption of the latest OS is pretty high. Then there is the problem of bloat and compatibility described in this post. In the meantime you can use Matter.js or some homebrew solution like discussed in this thread. I am computer free for the extended weekend! Okay got my phone 
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