Unwanted info warnings from EPUBCheck when TTF font files are used

[SteelWagstaff]

Issue: Our EPUB exports now generate validation warnings. These are technically not errors -- they're "Info" notes, which are defined by EPUBCheck as "A noteworthy information about your content." However, the message we display to users is "Warning: The export has validation errors. See logs for more details." See https://github.com/pressbooks/pressbooks/blob/51612fbe611a0d70883c588f0a9f4701f2a550de/inc/modules/export/class-export.php#L777-L838.

This repo bundles a lot of TTF files which are used in the font stacks of several of its child themes: https://github.com/pressbooks/pressbooks-book/tree/dev/assets/book/typography/fonts. These are referenced in the font stack SCSS partials: https://github.com/pressbooks/pressbooks-book/tree/dev/assets/book/typography/styles. See https://github.com/pressbooks/pressbooks-book/search?q=ttf.

We also include references to TTF files as part of a Pressbooks theme option: https://github.com/pressbooks/pressbooks/blob/9512bc006b5f193da3321a52cac451bcb747ec14/inc/class-globaltypography.php#L295-L422 and in a few other places/tests in that repo: https://github.com/pressbooks/pressbooks/search?q=ttf

Proposed solution:

1. Remove all unneeded TTF files (i.e. those used only by deprecated/retired themes): Remove fonts only used by retired themes #1120.
2. For those that remain, we should replace the TTF files with WOFF files. https://google-webfonts-helper.herokuapp.com/ can help generate the needed files. See https://www.oreilly.com/library/view/epub-3-best/9781449329129/ch04.html#how_to_embed for recommendations for how to embed fonts properly in EPUB3

Notes

Prince supports the use of WOFF or WOFF2 files: https://www.princexml.com/doc/styling#fonts

EPUB reader support for embedded fonts has been a mess for a long time. See https://www.oreilly.com/library/view/epub-3-best/9781449329129/ch04.html and https://www.oreilly.com/library/view/epub-3-best/9781449329129/ch04.html

The new EPUB3 spec now requires reading systems to support OpenType and WOFF font formats. See https://idpf.org/epub/30/spec/epub30-contentdocs.html#sec-css-fonts + https://idpf.org/epub/30/spec/epub30-changes.html#sec-new-changed-embedded-fonts

Browser support for OTF/TTF and WOFF is essentially interchangeable: https://caniuse.com/ttf + https://caniuse.com/woff
OTF files for the Noto fonts we use are available at https://github.com/googlefonts/noto-fonts/tree/main/unhinted/otf
On the difference between TTF and OTF: https://www.makeuseof.com/tag/otf-vs-ttf-fonts-one-better/
On the difference between TTF/OTF and WOFF: https://fonts.google.com/knowledge/glossary/web_font
See also https://www.news47ell.com/blog/host-google-fonts-locally-wordpress + https://www.news47ell.com/blog/host-google-fonts-locally-wordpress
Related issues: pressbooks/pressbooks#1629 + #752 + pressbooks/ideas#408 + pressbooks/pressbooks-book-directory-fe#423

Here's some sample log output:

Array
(
    [time] => Thu Jun  9 02:12:59 2022
    [user] => steel
    [site_url] => https://integrations.pressbooks.network/epubtest
    [blog_id] => 536
    [theme] => Malala
)
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(3,3): Font-face reference "EPUB/assets/EncodeSans-Regular.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(8,3): Font-face reference "EPUB/assets/EncodeSans-Bold.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(13,3): Font-face reference "EPUB/assets/EncodeSans-ExtraBold.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(18,3): Font-face reference "EPUB/assets/EncodeSans-Light.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(23,3): Font-face reference "EPUB/assets/Raleway-Bold.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(28,3): Font-face reference "EPUB/assets/Raleway-BoldItalic.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(33,3): Font-face reference "EPUB/assets/Raleway-SemiBold.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(38,3): Font-face reference "EPUB/assets/Raleway-SemiBoldItalic.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(43,3): Font-face reference "EPUB/assets/Raleway-Regular.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(48,3): Font-face reference "EPUB/assets/Raleway-Medium.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(53,3): Font-face reference "EPUB/assets/Raleway-MediumItalic.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(58,3): Font-face reference "EPUB/assets/Raleway-RegularItalic.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(63,3): Font-face reference "EPUB/assets/Raleway-Light.ttf" refers to non-standard font type "font/sfnt".
INFO(CSS-007): /srv/www/integrations.pressbooks.network/releases/20220608202328/web/app/uploads/sites/536/pressbooks/exports/EPUB-Test-1654740770.epub/EPUB/malala.css(68,3): Font-face reference "EPUB/assets/Raleway-LightItalic.ttf" refers to non-standard font type "font/sfnt".
See https://www.w3.org/publishing/epubcheck/docs/messages/

Font-face reference "%1$s" refers to non-standard font type "%2$s". Explanation: Font core-types are "TrueType", "OpenType", "WOFF" and "WOFF2"