Carlos contributed to the mozilla/build-a-b repository by focusing on front end development with React and TypeScript, delivering both a new feature and a bug fix within a month. He enhanced user experience by refining card navigation and modal workflows, introducing logic to auto-select the last played card by owner and improving modal layouts and button handling. Carlos also addressed cross-browser video playback reliability, particularly for Safari, by updating video tag handling in VSAnimation and ImageGallery components. His work emphasized type safety and maintainability, resulting in smoother user interactions and more robust media playback across different browsers and devices.