Example: Go to http://people.mozilla.com/~jkew/woff/udhr-sample/index.html.
Choose the 3rd option for display; it's a *large* font download, so you see a
delay. The text will stay invisible the entire time.
This effect is exacerbated on mobile devices and in low-bandwidth situations, preventing the user from seeing any text.
The solution that's been considered in www-style is to use a fallback font (whatever is next in the stack) after a few seconds if the font hasn't downloaded/isn't ready.
Associated mozilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=499292
More related detail:
*** This bug has been marked as a duplicate of bug 25207 ***