[en] 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→Wlν_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_F²m_t⁴), O(G_Fm_t²α_s), and O(G_Fm_t²α_s²). 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 τ lepton. (orig.)