Some web link solutions (for Epub3.0) below.
Epub2.0 won't allow CSS min/max, or calc(), or @supports, or @media. But with a bit of reading there you might find an alternate workable option. Moon+ App was a stumbling block even for me and involved additional CSS trickery (on a different CSS issue).
Simplified fluid font size:
CSS-tricks1
Viewport fluid font size:
CSS-tricks2