The Joint Photographic Experts Group (JPEG) is currently in the process of standardizing JPEG XL, the next generation image coding standard that o↵ers substantially better compression efficiency than existing image formats. In this paper, the quality assessment framework of proposals submitted to the JPEG XL Call for Proposals is presented in details. The proponents were evaluated using objective metrics and subjective quality experiments in three di↵erent laboratories, on a dataset constructed for JPEG XL quality assessment. Subjective results were analyzed using statistical significance tests and presented with correlation measures between the results obtained from di↵erent labs. Results indicate that a number of proponents superseded the JPEG standard and performed at least as good as the state-of-the-art anchors in terms of both subjective and objective quality on SDR and HDR contents, at various bitrates.