  Beginning with precise data on the ratio of structure functions in deep
inelastic scattering (DIS) from $^3$He and $^3$H, collected on the domain $0.19
\leq x_B \leq 0.83$, where $x_B$ is the Bjorken scaling variable, we employ a
robust method for extrapolating such data to arrive at a model-independent
result for the $x_B=1$ value of the ratio of neutron and proton structure
functions. Combining this with information obtained in analyses of DIS from
nuclei, corrected for target-structure dependence, we arrive at a prediction
for the proton's valence-quark ratio: $\left. d_v/u_v \right|_{x_B\to 1} =
0.230 (57)$. Requiring consistency with this result presents a challenge to
many descriptions of proton structure.
