The value is undefined if there are comment or processing instruction nodes. The current implementation returns false if it sees those, assuming the nodes haven't been stripped out during parsing. But I'm not married to the current implementation and reserve the right to change it without notice.
Added December 3, 2021 (dub v10.5)
Returns if the node would be printed to string as <tag /> or <tag></tag>. In other words, if it has no non-empty text nodes and no element nodes. Please note that whitespace text nodes are NOT considered empty; Html("<tag> </tag>").isEmpty == false.