We generalize the operational quasiprobability involving sequential measurements proposed by Ryu {em et al.} [Phys. Rev. A {bf 88}, 052123] to a continuous-variable system. The quasiprobabilities in quantum optics are incommensurate, i.e., they represent a given physical observation in different mathematical forms from their classical counterparts, making it difficult to operationally interpret their negative values. Our operational quasiprobability is {em commensurate}, enabling one to compare quantum and classical statistics on the same footing. We show that the operational quasiprobability can be negative against the hypothesis of macrorealism for various states of light. Quadrature variables of light are our examples of continuous variables. We also compare our approach to the Glauber-Sudarshan $mathcal{P}$ function. In addition, we suggest an experimental scheme to sequentially measure the quadrature variables of light.
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