Worked on enhancing test reliability and library compatibility for HTML parsing within the symfony/symfony repository. Focused on adapting PHPUnit-based tests to accommodate changes in libxml2 and updated HTML5 validity criteria, ensuring that test expectations accurately reflected the current behavior of the HTML parser. Addressed a release-critical bug by modifying tests to identify invalid HTML according to both HTML4 and HTML5 standards, which reduced the risk of false positives as dependencies evolved. Utilized PHP, HTML, and testing best practices to maintain the stability of the test suite, supporting ongoing development and safeguarding the integrity of HTML rendering components.