Does it tell you that align-items by itself doesn’t make sense regardless of the value, of the element isn’t otherwise set to use display: flex or grid?
I don't know the answer, but I'd be surprised if it did.
I've also been very frustrated by CSS due to 1) its lack warnings and debug errors as mentioned above, and 2) that so many properties have side-effects (like setting an element to display:flex changes the auto value of min-width for all its children[1]).
I've started working on something to help with this, but it's a long way off (and it may not work at all).