Create Wiki page, or documentation in code, describing how to test custom fonts

[benlk]

Description of the documentation inaccuracy

Brief description:

• write full description of how to specify custom font files, gathering bits and pieces from previous comments in issues
• write description of how to test custom fonts in Apple News without access to upload new fonts to the News Publisher backend, or without channel ID that matches
• document requirements for font files in local testing

This will help fill some gaps in the official Apple News docs regarding custom fonts.

Additional Information

Parts of this:

• example implementation of https://github.com/alleyinteractive/apple-news/wiki/Filter:-apple_news_fonts_list , see https://github.com/alleyinteractive/apple-news/issues/949
• "Any fonts you use that are not stock Apple News fonts need to exist in the same directory as the file." https://github.com/alleyinteractive/apple-news/issues/1137#issuecomment-2344550159
• the font files in the directory with article.json must have a file name that is the same as the PostScript font name
• font files must be in .otf
• recommended tools for inspecting .otf files: https://wakamaifondue.com/beta/ works
• caution on fonts with special features: if the font has features like oddball custom ligatures enabled by default, Apple News does not have controls to turn features off or on, unlike CSS
• recommended code patterns for handling local testing when custom fonts are in use in production, but the corresponding font files are not available for local testing.

Outstanding questions:

• is it possible within ANF to specify a URL to load a font from?

[kevinfodness]

With the most recent version of Publish to Apple News, any custom font files that are associated with your channel will automatically show up in the list of selectable fonts in the theme editor, which makes this process significantly easier. You won't be able to use custom fonts on your articles unless they are uploaded to your channel in any case, so it's a good idea to start by getting the custom fonts added to your channel.

[benlk]

The issue that we've encountered is that, though the fonts are uploaded to the channel, they are not available in News Preview.app, unless the fonts are either 1) installed via Font Book.app or 2) placed in the same folder as article.json.