Draft a page on polyfills (mdn#38326)

* Add a page on polyfills

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* Update files/en-us/mdn/writing_guidelines/page_structures/polyfills/index.md

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

* Update files/en-us/mdn/writing_guidelines/page_structures/polyfills/index.md

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

* Review feedback

---------

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

Author: wbamberg

files/en-us/mdn/writing_guidelines/page_structures/polyfills/index.md

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+---
+title: Polyfills
+slug: MDN/Writing_guidelines/Page_structures/Polyfills
+page-type: mdn-writing-guide
+sidebar: mdnsidebar
+---
+
+This page outlines MDN's policy for including polyfills in the reference documentation for [JavaScript](/en-US/docs/Web/JavaScript) and [Web APIs](/en-US/docs/Web/API).
+
+A {{glossary("Polyfill", "polyfill")}} is an implementation of a web platform feature that websites can use in browsers that do not natively support the feature. Polyfills enable web developers to write a single codebase targeting multiple browsers and browser versions, even when some of those browsers don't support some of the features used.
+
+Polyfills are important to web developers but are also a risk: buggy polyfills can break websites or create security vulnerabilities. For this reason, MDN recommends specific sources for polyfills, and is very conservative about adding additional sources.
+
+## Polyfills in the JavaScript reference
+
+### Selected polyfills
+
+The [JavaScript reference documentation](/en-US/docs/Web/JavaScript) can link to polyfills from two sources:
+
+- The [core-js](https://github.com/zloirock/core-js/tree/master) library.
+- The [es-shims](https://github.com/es-shims) organization.
+
+Sources are selected by the MDN maintainers based on the following criteria:
+
+- Conformance to the specification for the feature for which they are providing an implementation.
+- Adoption by the web development community, as seen in metrics such as the number of [npm](https://www.npmjs.com/) downloads.
+
+### Proposing an additional polyfill source
+
+Anyone can propose that MDN should recognize an additional source of polyfills by [starting a discussion in the MDN discussion forum](https://github.com/orgs/mdn/discussions). However, the MDN maintainers expect the number of recognized polyfills linked from MDN to remain very small, to reduce the risk of recommending polyfills that cause problems for web developers.
+
+### Integration of polyfills into pages
+
+When a page in the JavaScript reference documentation links to a polyfill, it adds the link in the "See also" section at the end of the page.
+
+The link is placed at the start of the "See also" list, in the following format:
+
+```md
+- [Polyfill for `featureName` in `project-name`](link)
+```
+
+## Polyfills in the Web API reference
+
+### Selected polyfills
+
+The [Web API reference documentation](/en-US/docs/Web/API) can link to polyfills that are maintained alongside the specification for the feature itself.
+
+For example, the [Trusted Types API](/en-US/docs/Web/API/Trusted_Types_API) has a polyfill which is [maintained in the same repository as the specification](https://github.com/w3c/trusted-types?tab=readme-ov-file#polyfill). Accordingly, the Trusted Types API reference documentation on MDN can link to that polyfill.
+
+### Integration of polyfills into pages
+
+Polyfills are typically integrated into the [overview page for the API](/en-US/docs/MDN/Writing_guidelines/Page_structures/Page_types/Page_type_key#web_api_page_types).