true if and only if this node is readonly.
The DOM specification defines a readonly node as "a node that is immutable. This means its list of children, its content, and its attributes, when it is an element, cannot be changed in any way. However, a read only node can possibly be moved, when it is not itself contained in a read only node."
EntityReferences and all of theirs descendants
are always readonly.
bool readonly() @property;
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