Worked on stabilizing DOM manipulation in the mozilla/gecko-dev repository by addressing a regression related to the moveBefore operation. Focused on ensuring that layout and style flags were preserved when elements were removed and reinserted, thereby reducing rendering inconsistencies and preventing stale state issues. Implemented a regression-safe fix in JavaScript and HTML, and contributed a Web Platform Test to verify style preservation after dynamic DOM updates. This work improved the reliability and performance of UI rendering, enhanced test coverage for edge cases, and maintained strong traceability through detailed commit references, supporting future audits and rollbacks within the codebase.