That's the problem of not having tests: you should have noticed that in Firefox vendor-prefixed style properties begin with "Moz", not "moz". Other suggestions: - one could pass the perfectly valid property "font-smoothing", the function would have tried searching for "webkitFont-smoothing"... and failed, because it's "webkitFontSmoothing". So, camelCase the *whole* property, or make the documentation clearer that they should use "fontSmoothing" instead; - it doesn't check for vendor-prefixed *values*, like "-ms-grid" for display. Unfortunately checking them isn't as easy, but that would be one of the perks of such a package. As it is, it's quite trivial and similar to many others (e.g.: https://www.npmjs.com/package/prefix-property).