vitorio
commented
7 years ago Every font is going to have its own license that says what one can and cannot do with it. That may not include the permission to use it as a web font, which may require a separate license.
Fonts are software, and are licensed using licenses that describe the permitted types of use, just like software. Fonts on Google Fonts are each licensed under their own licenses, and it's conceivable a given font's license may allow Google to host them, but not other sites.
Usually, the fonts included with one's operating system are licensed for use as part of the OS and in content one creates, where you render the font out (like images or printing), but not for redistributing the font code itself (converting to a web font format). (But, not necessarily, some fonts might only be licensed for the OS to use to render text elements and not the end-user!)
Here's Microsoft's font FAQ: https://www.microsoft.com/typography/faq/faq11.htm
The fonts are governed by the same restrictions as the products they are supplied with. You are not allowed to copy, redistribute or reverse engineer the font files. For full details see the license agreement supplied with the product.
Here's macOS Sierra's EULA, section E spells out the font licensing: http://images.apple.com/legal/sla/docs/macOS1012.pdf
Subject to the terms and conditions of this License, you may use the fonts included with the Apple Software to display and print content while running the Apple Software; however, you may only embed fonts in content if that is permitted by the embedding restrictions accompanying the font in question. These embedding restrictions can be found in the Font Book/Preview/Show Font Info panel.
Individual fonts one might license from a foundry probably distinguish their licensing between desktop (personal publishing), server (generating renders for others), embedded (e.g. bundling with a product) and webfont (online publishing) usage. Here's an example from MyFont, which resells from many foundries: http://www.myfonts.com/licensing/
I've personally licensed Century Schoolbook from them, as a web font, and my license is by the page view: http://www.myfonts.com/fonts/bitstream/century-schoolbook/#family/49 (click on "Buying choices" in the upper right). Incidentally, they also provided all the CSS and font files I needed to self-host, I didn't need to convert or process anything.
The legal issues around distributing fonts are foreign to me. I'm assuming anything on Google Fonts is fine. But for fonts from other sources...?