This simple style sheet

nobr
{white-space:nowrap;}
wbr:before
{content:"\200B";
white-space:normal;}

makes <nobr> and <wbr> custom elements to work in Prince 5 as expected:
http://www.utoronto.ca/webdocs/HTMLdocs/NewHTML/wbr.html

<nobr>
Line Breaks Are Allowed Only At Word Break Tag<wbr/>
Line Breaks Are Allowed Only At Word Break Tag<wbr/>
Line Breaks Are Allowed Only At Word Break Tag<wbr/>
Line Breaks Are Allowed Only At Word Break Tag
</nobr>