Added 'size-adjust' descriptor for @font-face rules

features/css-size-adjust-descriptor.md

@@ -0,0 +1,9 @@
+---
+title: CSS size-adjust descriptor for @font-face rules
+category: css, layout
+bugzilla: 1698495
+firefox_status: in-development
+spec_url: https://drafts.csswg.org/css-fonts-5/#size-adjust-desc
+---
+
+Allow to adjust the scaling of individual fonts loaded using @font-face, to better harmonize the visual sizes of different designs, or to more closely match a fallback face to a resource that may be swapped in later, thereby minimizing visual layout shift when the final font becomes available.