Inthecontextofthe[AccessibilityforReadingproject](https://www.mediawiki.org/wiki/Reading/Web/Accessibility_for_reading) we needed a way to enable font-size customization in Vector in a scalable way. CSS custom properties were the mechanism chosen to enable this functionality. Full discussion available on [T346072](https://phabricator.wikimedia.org/T346072).
CSSCustompropertieswillbemarkedas`@private`and`@experimental`inthespiritofthe[Stableinterfacepolicy/Frontend](https://www.mediawiki.org/wiki/Stable_interface_policy/Frontend) in order to discourage usage outside of Vector. This could be changes to `@internal` if usage outside of Vector is deemed necessary.