Φ cannot be displayed, but φ can. Why?

Some Greek letters here cannot be displayed.

droid_sans_regular.typeface.json is used. Is that particular font not good enough?

It may also be a browser problem. For example, I cannot even write those two letters here properly. I write one “phi” on the left pane here, it will be displayed as another “phi” on the right pane.

I’m not sure this forum is the ideal place for such a typeface.js specific question. You probably get more response by posting at stackoverflow.

All the glyphs are visible in the json file. Have you checked to see whether these characters are in the file?