The ongoing searches for Higgs-boson signals in data taken at the CERN LHC and the Fermilab Tevatron crucially rely on the decay channels H -> Zll and H -> Wl nu(l). We present a precision study of the partial widths of these decay channels including the full one-loop electroweak corrections and the dominant contributions at two and three loops, of O(G(F)(2)m(t)(4)), O(G(F)m(t)(2)alpha(s)), and O(G(F)m(t)(2)alpha(s)). Since the invariant mass of the off-shell intermediate boson is relatively low in the mass window 115 GeV < m(H) < 129 GeV of current interest, lepton mass effects are relevant, especially for the tau lepton.