Mapnik uses Cairo or AGG for rendering (I guess one can choose) . Both are able to handle the same functions with another output (PNG, SVG, PDF, …)
Osmarender uses XSLT. Therefore the OSM-XML is directly transoformed into an SVG (which ist XML as well). After that Tiles@Home (which produces the osmarender-layer) uses Inkscape  for rendering PNG-tiles out of the SVGs.
Probably OpenLayers can help here:
As far as I know a Vector Layer in OpenLayers is embedded SVG as well. It can get its data directly from osm-files, geojson, and a couple of other formats and has options for styling, mouse events and so on.
EDIT: Look at this example: http://openlayers.org/dev/examples/osm-layer.html
it retrieves its data from a (in this case cached) osm-file, and on click it also displays the tags of the element. As the tags are saved in variables it shouldn’t be any problem to fill text-input-fields with it.