Developed comprehensive XPath id() function tests for the mozilla/gecko-dev repository, focusing on enhancing web-platform-tests coverage as part of ongoing XPath core function work. The approach involved creating detailed test cases in JavaScript and HTML to verify single and multiple ID lookups, handling of non-existent and duplicate IDs, case sensitivity, and IDs containing special characters or whitespace. This work strengthened cross-engine regression detection within the WPT suite, supporting earlier identification of XPath id() related issues. Documentation and test patterns were aligned with gecko-dev guidelines, improving maintainability and easing onboarding for future contributors while supporting the broader QA strategy.