Anomaly of strings of 6d $\mathcal{N}=(1,0)$ theories

We obtain the anomaly polynomial of strings of general 6d $\mathcal{N}=(1,0)$ theories in terms of anomaly inflow. Our computation sheds some light on the reason why the simplest 6d $\mathcal{N}=(1,0)$ theory has $E_8$ flavor symmetry, and also partially explains a curious numerology in F-theory.