Serre fibration and CW-complexes

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Suppose that $p:X\rightarrow E$ is a Serre fibration. I know the definition: then $p$ has the right lifting property with respect to all inclusions $I^n\rightarrow I^{n}\times I$. Now it seems that one can equivalently say that one has the right lifting property with respect to all CW-complexes. One implication is clear. But why is the from $I^n$ to CW-complexes correct. Comes this from the pushout construction of CW-complexes?