Essentially, what you want from a polyfill delivery system is a way of sending one single copy of each of the best polyfills, in the right order (respecting dependencies), to just the browsers that lack a native implementation. The polyfill service does this by reading the User-Agent HTTP header and then using it to find the list of polyfills that are suitable for that browser and its specific version. Typically once a browser version is released, its feature set does not change until its next release.
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