Font Availability Checker

In short There is no browser API that lists installed fonts — that would be a fingerprinting gift — so availability is detected indirectly. The technique is to render a string in the font you are testing with a known fallback behind it, and measure the width: if the text is the same width as the fallback, the font was not found and the fallback drew it; if the width differs, the font exists. It is a trick rather than an interface, and it is what every tool of this kind uses. That also explains why Safari reports so few fonts. Apple deliberately restricts font enumeration to a fixed allowlist to limit fingerprinting, so a Mac with hundreds of fonts installed will report only the standard set, and a negative result there means "not visible to this browser" rather than "not installed". The practical use is debugging a CSS font stack: when a page looks wrong on one machine and right on another, the usual cause is a font in the stack that exists in one place and not the other, and the browser silently fell through to the next entry. Knowing which entry actually rendered turns a mystery into a one-line fix.