This guide outlines the process of creating and implementing Web Platform Tests (WPT) for a new CSS feature within Chromium's Blink engine. It details steps from initial feature implementation under a runtime flag to designing comprehensive tests covering parsing, computed style, rendering, inheritance, and dynamic changes. The document further explains how to author test files using testharness.js, run and debug these tests locally within the Chromium environment, and eventually upstream them to the official WPT repository for cross-browser verification. Crucially, it emphasizes adherence to WPT guidelines for interoperability and maintaining high-quality, reliable tests.