  We extend the covariant, parametrized post-Friedmann treatment of cosmic
acceleration from modified gravity to an arbitrary admixture of matter,
radiation, relativistic components and spatial curvature. Explicit expressions
in the comoving, Newtonian and synchronous gauges facilitate the adaptation of
Einstein-Boltzmann codes for solving CMB and matter perturbations in the linear
regime. Using a comoving gauge code, we study the effect of metric evolution on
the CMB through the integrated Sachs-Wolfe effect. Modified gravity can alter
the low multipole spectrum, including lowering the power in the quadrupole.
From a principal component description of the primary metric ratio parameter,
we obtain general constraints from WMAP on modified gravity models of the
acceleration.
