Unicode values
Left, symbols from Allumi and their Unicode values. Right, a symbol from Freight Sans which has no Unicode value and cannot be used safely on the web.
Tim Ahrens for Typekit:
Not only should the website look like we want it to, we also need to make sure the underlying code is standards-compliant so that search engines, screen readers, translation tools, or web archives can make sense of the text. This is why we make sure that each “drawer” contains what it says on the label, so as not encourage font use that may look like it should but contradicts the principles of the web.