XML DOM lost spaces in QDomText/HTML

When I parse text data from xml like:
<pp PX="0" CX="0">
<tp CX="1">Hello</tp>
<tp CX="0"> </tp>
<tp CX="2">Moon</tp>
</pp>
In second tp node, there are three spaces, but in XML Dom, it's empty.
QDomElement tpNode,
tpNode.firstChild().toText().isNull() is true.

This issue also exists in html parse.
If use QTextDocument.setHtml("<p>Hello</p><p> </p><p>Moon</p>"),
you will find there only two lines.

All the spaces are ignored, maybe I missed something, anybody can help me?