font-size-adjust two value syntax (Error: Too many values)

[1jj]

Two-value syntax of font-size-adjust returns an error, It is supported in all major engines. Single value is fine.

foo { font-size-adjust: 0.5 }
No error here. This is equivalent:

foo { font-size-adjust: ex-height 0.5 }
Value Error: font-size-adjust Too many values or values are not recognized : ex-height 0.5