We changed DOM4 so that ProcessingInstruction is much more like Text and Comment.
This seems to have regressed updating the contents of a PI and having that restyle the document - but re-inserting the PI after changing its contents offers a workaround. Is this expected behaviour (maybe the API we're supposed to use changed?), or should I file a bug to fix this in core? See bug 888672 for more details and easy STR.
Please file a bug. It seems that it might be caused by pi->SetData now calling nsGenericDOMDataNode::SetData -> SetTextInternal directly rather than SetNodeValue -> SetNodeValueInternal which XMLStylesheetProcessingInstruction overrides.
Yes, new bug please.