The Polymer Project is a library for creating web components that align with W3C standards, making it easier to create custom HTML elements. The testing framework for the Polymer project is called Web Component Tester (WCT), which wraps around Mocha, Chai, Sinon, Selenium, and more, running test suites through browsers using Selenium and integrating with Sauce Labs for remote execution. Custom web components enable new types of DOM elements and Shadow DOM combines multiple DOM trees into one hierarchy, enabling better composition of the DOM. The Polymer project uses polyfills to support a range of browser versions and has a command-line interface (CLI) tool that includes a test runner WCT, providing a comprehensive solution for running unit tests inside real browsers. A solid testing strategy for Polymer projects requires a foundation of unit tests for web components, integration testing with remote data, automated responsive/adaptive integration testing, functional end-to-end testing, and visual checkpoints using Applitools.