See other not-so-valid CSS things
:stuck { ... }
:stuck { ... } is an invalid CSS declaration I wish existed."
border-color: gradient(...)
border-color: gradient(...) is an invalid CSS declaration I wish existed.
collapsing: none
collapsing: none is an invalid CSS declaration I wish existed.
place: center
place: center is an invalid CSS declaration I wish existed.
font-family: system-font
font-family: system-font is an invalid CSS declaration I wish existed.
position-items: center
position-items: center is an invalid CSS declaration I wish existed.
position: temporal
position: temporal is an invalid CSS declaration I wish existed.
color: anthracite
color: anthracite is an invalid CSS declaration I wish existed.
float: top
float: top is an invalid CSS declaration I wish existed.