Typically, DOM operations patched in this polyfill gather the list of elements to which a given callback would apply and then iterate that list, calling the callback on each element. WebViewer has already polyfilled Symbol with a polyfill using native code in the iframe. The polyfill gracefully enhances the following minimum versions of at least these browsers, up to their latest version: Either install this module via npm i @ungap/custom-elements, and include it in your project, or use a CDN such as unpkg.com to obtain the minified version of this module. custom-elements-es5-adapter.js. To get an instance of it, use the window.customElements property. It does this as a performance optimization. Custom Elements v1 polyfill with built-in components support Promise polyfill for IE11 support CustomEvent polyfill for IE11 support ResizeObserver polyfill to detect element size changes Element closest(), matches(), remove This package uses the shims from the webcomponents repo and combines them into a single polyfill that can be added to your project with npm. We are actively working on migrating open Issues and PRs to the new repo. It seems that Polymer's polyfill-next-selector { content: ':host #myId' } Does not work (in IE) for VanillaJS