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{{other uses|Polyfill (disambiguation)}}
In [[software development]], a '''polyfill''' is code that implements a feature of the [[development environment]] that does not natively support the feature. Most often, it refers to [[JavaScript]] code that implements an [[HTML5]] or [[CSS]] [[web standard]], either an established standard (supported by some browsers) on older browsers, or a proposed standard (not supported by any browsers) on existing browsers. Polyfills are also used in [[PHP]] and [[Python (programming language)|Python]].<ref>{{Cite book |last=Thormeier |first=Pascal |url=https://books.google.com/books?id=yprEEAAAQBAJ&dq=polyfill+php&pg=PA195 |title=Mastering CSS Grid: A comprehensive and practical guide to creating beautiful layouts with CSS Grid |date=2023-06-09 |publisher=Packt Publishing Ltd |isbn=978-1-80461-616-1 |pages=195 |language=
Polyfills allow web developers to use an API regardless of whether or not it is supported by a browser, and usually with minimal overhead. Typically they first check if a browser supports an API, and use it if available, otherwise using their own implementation.<ref name="speakingjs"/><ref>"It typically checks if a browser supports an API. If it doesn’t, the polyfill installs its own implementation. That allows you to use the API in either case."</ref> Polyfills themselves use other, more supported features, and thus different polyfills may be needed for different browsers. The term is also used as a verb: ''polyfilling'' is providing a polyfill for a feature.
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