I have some CSS that looks like:

@page {
  size:6in 9in;
  margin: 2cm;
  font-size: 9pt;
  orphans:3; 
  widows:3;  
}

@page chapter:right
{
  @top-right
  {
    content: string(section-title, last) "\A0\A0\002D\A0\A0" counter(page);
    font-family: Helvetica, Arial, sans-serif;  
    vertical-align: right;
  }
}

.front-matter-title-wrap > h1:first-of-type, 
div.chapter > h2:first-of-type, 
div.back-matter > h1:first-of-type {
    string-set: section-title content();
    /* ... */
}

This works. Our title is carried over to where we want it, as a string. Horray!

But now we have a case where we want to carry over the style too. Example title would look like "My <em>fancy</em> title" and we want to preserve the <em> tags.

I looked at a CSS spec that says running elements can do it?

http://www.w3.org/TR/css3-gcpm/#running-elements

I tried replacing:

 content: string(section-title, last) "\A0\A0\002D\A0\A0" counter(page);

to:

content: element(section-title, last) "\A0\A0\002D\A0\A0" counter(page);

( and string-set: section-title content() ... )

Clearly a dumb brute force attempt that didn't do anything. But, I'm sort of stuck.

Any ideas?

Prince supports running elements using our own "flow" mechanism described here under "Taking elements from the document".