Why does `transition: --hue 1s;` smoothly animate a hue value only when `--hue` is registered via `@property` with `syntax: '<angle>'`, but jump discretely (or not transition) when it is an unregistered custom property?
@property --hue { syntax:'<angle>'; inherits:false; initial-value:0deg; }
.box { --hue: 0deg; transition: --hue 1s; }
.box:hover { --hue: 120deg; }