Some of the code in MarkupAccumulator would rely on the node's document to determine whether or not to use the XML or HTML serialization algorithm instead of using the serialization algorithm passed to the MarkupAccumulator constructor. This is often fine but leads to behavior changes when the node is a <template> since they have their own template document.

This meant that we may start serialization as HTML and then switch to XML for serialization a particular template node.