set(decl, prefix) {
    let spec = flexSpec(prefix)[0]
    if (spec === 2012) {
      decl.value = AlignContent.oldValues[decl.value] || decl.value
      return super.set(decl, prefix)
    }
    if (spec === 'final') {
      return super.set(decl, prefix)
    }
    return undefined
  }
Change value for 2012 spec and ignore prefix for 2009