...yes, that's what a polyfill is: a javascript implementation of a new spec that's only applied when the current browser doesn't yet support the new spec. This lets devs start using it right away, then when it has enough support across browsers the polyfill can be removed without changing their code.
Very curious to know more about this, regarding PMTiles:
> Unfortunately, making range requests in 80 GB files just doesn't work in production. It is fine for files smaller than 500 MB, but it has terrible latency and caching issues for full planet datasets.
Technically why it's slow is entirely on CloudFlare, so I can only guess. But probably they are not mirroring files of this size across their datacenters / servers. I don't know how their public buckets hosting is done, but it's definitely very slow.
OpenFreeMap on the other hand seems almost instant for me.
No, it uses Mastodon's API. Can't log in with Threads, but you can follow some Threads account from Mastodon; "Known Limitations" section here https://www.augment.ink/threads-on-mastodon/
HTML5 Shiv was needed, with added history https://www.paulirish.com/2011/the-history-of-the-html5-shiv... (2011)