float: top
Have you ever wondered why CSS has no top and bottom float values?
'top middle' and 'bottom middle' sound great.
See other not-so-valid CSS things
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.