  We consider the time-harmonic Maxwell equations posed in $\mathbb{R}^3$. We
prove a priori bounds on the solution for $L^\infty$ coefficients $\epsilon$
and $\mu$ satisfying certain monotonicity properties, with these bounds valid
for arbitrarily-large frequency, and explicit in the frequency and properties
of $\epsilon$ and $\mu$. The class of coefficients covered includes (i) certain
$\epsilon$ and $\mu$ for which well-posedness of the time-harmonic Maxwell
equations had not previously been proved, and (ii) scattering by a penetrable
$C^0$ star-shaped obstacle where $\epsilon$ and $\mu$ are smaller inside the
obstacle than outside. In this latter setting, the bounds are uniform across
all such obstacles, and the first sharp frequency-explicit bounds for this
problem at high-frequency.
