I followed to

http://princexml.com/bb/viewtopic.php?t=1078

to create a TOC.

The section numbers as defined through a h1, h3, h4 structure is inserted before the <hX> tags using

.l1 h1:before {
content: counter(chapter) "." counter(section) " ";
}

.l1 h3:before {
content: counter(chapter) "." counter(section) "." counter(subsection) " ";
}

.l1 h4:before {
content: counter(chapter) "." counter(section) "." counter(subsection) "." counter(subsubsection) " ";
}

How can I insert the same chapter/section numbers into the TOC?

Have you tried using target-counter(attr(href), chapter) target-counter(attr(href), section), and so on?