Worked on improving DASH media playback reliability in the shaka-player repository by addressing a bug related to Representation ID substitution in segment URIs when supplemental codecs are present. The solution involved ensuring that the correct originalId is used for URI substitution, which resolved 404 errors encountered during segment fetching. Automated tests were added to verify the new behavior, enhancing maintainability and traceability of the fix. This work, implemented in JavaScript with a focus on DASH streaming and media playback, contributed to more robust handling of supplemental codecs and improved the overall user experience for DASH streams in the shaka-player project.