  We examine spectral properties of the ionising continua, the Lyman-break, and
the Ly-a and HeII 1640 recombination lines in starbursts. The transition from
primordial galaxies to currently observed metallicities, is examined. For the
average properties of starbursts, the main findings are: 1) The Lyman continuum
flux increases with decreasing metallicity. For a universal Salpeter IMF from
1-100 Msun the enhancement reaches typically a factor of ~3 between solar
metallicity and Pop III objects. 2) While for metallicities Z >~ 1/50 Zsun the
amplitude of the Lyman-break depends little on Z, a reduction by a factor ~2 is
found at lower Z, due to the strong increase of the average stellar
temperature. 3) Using theoretical models and empirical constraints we discuss
the expected evolution of the hardness of He+ to H ionising photons with
metallicity and possible uncertainties. We also provide a simple estimate of
the possible impact of hot WR like stars on Q(He+)/Q(H) at very low
metallicities. 4) Calibrations for star formation rate determinations from
various recombination lines at all metallicities and for various IMFs are
derived.
  For young bursts the maximum Ly-a equivalent width is shown to increase
strongly with decreasing metallicity for the same IMF. However, for well known
reasons, the Ly-a emission predicted likely represents an upper limit.
Non-negligible HeII 1640 emission due to stellar photoionisation appears to be
limited to very low metallicities and Population III objects.(abridged
abstract)
