That's how they used to work. However, it was just as jarring as the OP's complaint: the flash of unstyled content. You'd start reading something while the font

That's how they used to work. However, it was just as jarring as the OP's complaint: the flash of unstyled content. You'd start reading something while the fonts downloaded. Suddenly, the whole page would jump and change as the new font (with its metrics, kerning, etc) are applied. IIRC, Firefox waits a few seconds showing no text. If the font still isn't available, it resorts to fallback fonts and redraws the page if/once the font is actually downloaded.