  Experimental form factors of the hydrogen and helium isotopes, extracted from
an up-to-date global analysis of cross sections and polarization observables
measured in elastic electron scattering from these systems, are compared to
predictions obtained in three different theoretical approaches: the first is
based on realistic interactions and currents, including relativistic
corrections (labeled as the conventional approach); the second relies on a
chiral effective field theory description of the strong and electromagnetic
interactions in nuclei (labeled $\chi$EFT); the third utilizes a fully
relativistic treatment of nuclear dynamics as implemented in the covariant
spectator theory (labeled CST). For momentum transfers below $Q \lesssim 5$
fm$^{-1}$ there is satisfactory agreement between experimental data and
theoretical results in all three approaches. However, at $Q \gtrsim 5$
fm$^{-1}$, particularly in the case of the deuteron, a relativistic treatment
of the dynamics, as is done in the CST, is necessary. The experimental data on
the deuteron $A$ structure function extend to $Q \simeq 12$ fm$^{-1}$, and the
close agreement between these data and the CST results suggests that, even in
this extreme kinematical regime, there is no evidence for new effects coming
from quark and gluon degrees of freedom at short distances.
