Matej Latin takes us through the current state of fluid typography on the web:
Fluid typography gives us so many opportunities to better design the reading experiences on the web but, at the same time, it introduces problems of font sizes scaling uncontrollably and potential accessibility issues. Is fluid web typography ready to be used?
TLDR: No, it’s not ready. But you should still read the article, which has a great breakdown of pros and cons of fluid typography and other typography approaches.