  All (in)homogeneous bond percolation models on the square, triangular, and
hexagonal lattices belong to the same universality class, in the sense that
they have identical critical exponents at the critical point (assuming the
exponents exist). This is proved using the star-triangle transformation and the
box-crossing property. The exponents in question are the one-arm exponent
$\rho$, the 2j-alternating-arms exponents $\rho_{2j}$ for $j\ge1$, the volume
exponent $\delta$, and the connectivity exponent $\eta$. By earlier results of
Kesten, this implies universality also for the near-critical exponents $\beta$,
$\gamma$, $\nu$, $\Delta$ (assuming these exist) for any of these models that
satisfy a certain additional hypothesis, such as the homogeneous bond
percolation models on these three lattices.
