set(decl, prefix) {
    let spec = flexSpec(prefix)[0]
    if (spec === 2009) {
      decl.value = list.space(decl.value)[0]
      decl.value = Flex.oldValues[decl.value] || decl.value
      return super.set(decl, prefix)
    }
    if (spec === 2012) {
      let components = list.space(decl.value)
      if (components.length === 3 && components[2] === '0') {
        decl.value = components.slice(0, 2).concat('0px').join(' ')
      }
    }
    return super.set(decl, prefix)
  }
Spec 2009 supports only first argument
Spec 2012 disallows unitless basis