.em works only in firefox?

In css I want padding: 21.5px - this works only FF.

I tried to use .em 1.68em and works only FF     padding: 3px 1.68em;

The problem  is because I want to move for more then 0.5 px. 21px this is not good for me.

How to use 21.5px should that works for all browsers?

Regards
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