I've been using the following to increase certain font sizes on phones.
@media only screen and (max-device-width : 720px)
It works on every phone and browser I've tried except Firefox on Nexus 4. It does work on Chrome on Nexus 4 as well as Firefox on Galaxy Nexus. If I increase it to 768px it works on Firefox on Nexus 4 but I think that will trigger iPads too.
Has anyone found a good way to target only phones?