Both Polymer v2 (previous version) and Polymer v3 (current version) are based on Shadow DOM v1, as can be seen in the docs (https://www.polymer-project.org/3.0/

Both Polymer v2 (previous version) and Polymer v3 (current version) are based on Shadow DOM v1, as can be seen in the docs (https://www.polymer-project.org/3.0/docs/devguide/shadow-dom https://www.polymer-project.org/3.0/docs/devguide/shadow-dom, https://www.polymer-project.org/2.0/docs/devguide/feature-overview https://www.polymer-project.org/2.0/docs/devguide/feature-ov...) so this statement is VERY strange. Although it's true that only Safari and Chrome (through WebKit and Blink) currently have the best support for Web Components, and polyfills are required for everything else due to flaky support. Google even mentions this in the Polymer README: "Web Components are now implemented natively on Safari and Chrome (~70% of installed browsers), and run well on Firefox, Edge, and IE11 using polyfills. Read more below." Source: https://github.com/Polymer/polymer https://github.com/Polymer/polymer