Is the order of two siblings implementation dependent?

If I have an XML document like this one:

<people>
    <person>
        <name> Mario </name>
        <age> 25 </age>
    </person>
    <person>
        <name> Lucas </name>
        <age> 32 </age>
    </person>
</people>

Is always guaranteed that in the document order, Mario occurs before Lucas? so Lucas is always a following sibling of Mario, or it's implementation dependent?