To best of my knowledge, not all browsers support the format, so, unfortunately,
you cannot do it.
To some extent, in principle, you can map some XPS data to "plain" HTML (I don't know why did you thing that HTML5 can help, specifically) and "export" XPS to HTML. In principle, this is possible just because XPS specification is open; this is the ECMA-388 standard:
http://en.wikipedia.org/wiki/Open_XML_Paper_Specification[
^],
http://www.ecma-international.org/publications/standards/Ecma-388.htm[
^].
As you can get full description of the format, you can do it, but this is just some particular mapping, not one-to-one correspondence. The approaches of these formats are very different. HTML is a flow document, XPS is very rigid, contains the alignment of graphics attached to the pieces of paper. However, as most XPS documents are actually originated from flow documents (such as Word, LibreOffice and a lot more), such "export" would be successful in very many cases.
Unfortunately, from the first attempt, I could not find any open-source ready-to-use solutions, but you can try to find something:
http://bit.ly/WHI3lW[
^].
—SA