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Currently the default value is fraction, but IMHO pixels is more commonly used.
The benefit of the current defaults is that they should give decent results with a variety of icon sizes.
An alternative would be to parse user provided values and derive the units (as is done with things like 20px and 50% in CSS).
good point @tschaub hadn't thought about that
@bartvde, what are your current thoughts on this?
I think we close this and leave it as is.