9 speed are just slightly narrower, the spacing is the same on the chain rings, but a 9 speed cassette is the same with as a 8 speed, so the cogs and chain are narrower to fit 9 in the same place as 8...
Using 8 speed rear with 9 speed fron is less of a problem than the other way...
But shifting will be less crisp, if you mix it up.