polyfill.ioは安全ですか？

This domain “reads the User-Agent header of each request and returns polyfills that are suitable for the requesting browser. Tailor the response based on the features you're using in your app, and see our live examples to get started quickly.” Source at https://polyfill.io/v2/docs/ If you are not familiar with “polyfills”, it is code that implements a feature on older web browsers that do not support the feature. Such as JavaScript library that implements an HTML5 web standard. Read more about polyfills at ***** The polyfill.io “service is developed and maintained by a community of contributors led by a team at the Financial Times.” Source at https://polyfill.io/v2/docs/ The polyfill.io's code is more secure because it can be review publicly at *****

Apparently it is a service maintained by Financial Times staffers that helps to boost content on outdated browsers. Assuming that everything is clean(I didn't see anything to indicate otherwise) the only issue is running ancient browsers usually is a bad idea.

From the site: "The polyfill service selectively bundles browser polyfills based on the User-Agent header supplied in a request, with the intention of allowing JavaScript and CSS developers to use modern standards in legacy user agents that do not natively support the standard. Think of it like a pair of glasses for your aging browser."