Introducing Font Face, a server-side `@font-face` styles generator and printer.
tl;dr:
* Introduces Font Face.
* Deprecates `_wp_theme_json_webfonts_handler()`.
**Introduce Font Face**
From an array of fonts (i.e. each font-family and its font variations to be processed), it:
1. Validates each `font-face` declaration, i.e. the CSS property and value pairing. If validation fails, processing stops with no font-face styles printed.
3. Generates the `@font-face` CSS for each font-family.
4. Prints the CSS within a `<style id="wp-fonts-local">` element.
The entry point into Font Face is through a new global function called `wp_print_font_faces()`, which is automatically called:
* when the `'wp_head'` hook runs (for the front-end).
* when the `'admin_print_styles'` hook runs (for the back-end).
* when `_wp_get_iframed_editor_assets()` runs to inject the `@font-face` styles into the iframed editor.
Once called, it gets the fonts from Theme_JSON merged data layer, which includes theme defined fonts and user activated fonts (once the Font Library #59166 is introduced into Core).
For classic sites, themes and plugins can directly call `wp_print_font_faces()` and pass their fonts array to it for processing.
**Deprecates `_wp_theme_json_webfonts_handler()`.**
As Font Face is a direct replacement, the stopgap code in `_wp_theme_json_webfonts_handler()` (introduced in 6.0.0 via [53282]) is deprecated and unused in Core.
**Props note:**
There's a long multiple year history baked into Font Face, which dates back to the early versions of a web font API (see #46370 and [https://github.com/WordPress/gutenberg/issues/41479 roadmap]. The props list includes those who contributed from those early versions up to this commit.
**References:**
* #46370 original (Web)Fonts API proposal for registering and enqueuing web fonts.
* [https://github.com/WordPress/gutenberg/issues/41479 Gutenberg tracking issue] which includes the evolution from Webfonts API to Fonts API to Font Face.
* [53282] / #55567 Added the stopgap code `_wp_theme_json_webfonts_handler()` in 6.0.
* [https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face @font-face on mdn web docs]
* #59166 Font Library: Font manager for WordPress
Follow-up to [53282].
Props aristath, jonoaldersonwp, hellofromTonya, andraganescu, annezazu, antonvlasenko, arena, askdesign, azaozz, bph, bradley2083, colorful-tones, costdev, davidbaumwald, desrosj, dingo_d, djcowan, domainsupport, dryanpress, elmastudio, flixos90, francina, garrett-eclipse, gigitux, grantmkin, grapplerulrich, gziolo, ironprogrammer, jb510, jeffpaul, jeremyyip, jffng, joostdevalk, jorgefilipecosta, juanmaguitar, mamaduka, matveb, mburridge, mitogh, ndiego, ntsekouras, oandregal, ocean90, oglekler, paaljoachim, pagelab, peterwilsoncc, poena, priethor, scruffian, SergeyBiryukov, shiloey, simison, skorasaurus, soean, westonruter, wildworks, zaguiini.
Fixes#59165.
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