Relative permittivity shows that force of interaction in some media is 3 times less than in vacuum ($\varepsilon = 3$) for example. When looking tables of permittivity I never saw a value less than 1.
Russian wikipedia says that $\varepsilon > 1$ is typical for low frequency. So I guess that under some conditions it is possible that $\varepsilon < 1$.
For a static (electrostatic) electrical field is it ever possible (and for what material) that $\varepsilon < 1$?
For the electric component of EMF field - when is it possible that $\varepsilon < 1$?