Comment 3 for bug 1708138

> this would mean that the _ProcessingInstruction object might have to remember whether it is an XML or an SGML PI?

Yes. In fact, its document would know (-> document node type), but for an independently created PI, there is no indication which type was intended (it would always be an XML PI). And if you parse a PI from an HTML document and then stick it into an XML tree, you'd still end up with the same problem, even on serialisation.

It would probably be possible to change the text content when moving PIs between XML and HTML documents, but then it would also have to be done recursively...

Seems like one of those quirks that are best documented somewhere and otherwise forgotten. ;)