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We present the first characterization of the spectral properties of superradiant light emitted from the ultranarrow, 1-mHz-linewidth optical clock transition in an ensemble of cold ^{87}Sr atoms. Such a light source has been proposed as a next-generation active atomic frequency reference, with the potential to enable high-precision optical frequency references to be used outside laboratory environments. By comparing the frequency of our superradiant source to that of a state-of-the-art cavity-stabilized laser and optical lattice clock, we observe a fractional Allan deviation of 6.7(1)×10^{-16} at 1 s of averaging, establish absolute accuracy at the 2-Hz (4×10^{-15} fractional frequency) level, and demonstrate insensitivity to key environmental perturbations.