  We present a microscopic theory for the nonequilibrium interfacial tension
$\gamma_{\rm gl}$ of the free interface between gas and liquid phases of active
Brownian particles. The underlying square gradient treatment and the splitting
of the force balance in flow and structural contributions is general and
applies to inhomogeneous nonequilibrium steady states. We find $\gamma_{\rm
gl}\geq 0$, which opposes claims by Bialk\'e et al. [Phys. Rev. Lett.
$\bf{115}$, 098301 (2015)] and delivers the theoretical justification for the
widely observed interfacial stability in active Brownian dynamics many-body
simulations.
