  Static disorder in a noninteracting gas of electrons confined to two
dimensions can drive a continuous quantum (Anderson) transition between a
metallic and an insulating state when time-reversal symmetry is preserved but
spin-rotation symmetry is broken. The critical exponent $\nu$ that
characterizes the diverging localization length and the bulk multifractal
scaling exponents that characterize the amplitudes of the critical wave
functions at the metal-insulator transition do not depend on the topological
nature of the insulating state, i.e., whether it is topologically trivial
(ordinary insulator) or nontrivial (a $Z_2$ insulator supporting a quantum spin
Hall effect). This is not true of the boundary multifractal scaling exponents
which we show (numerically) to depend on whether the insulating state is
topologically trivial or not.
