Every node has a parent (a node higher up the tree that it is a child of), and possibly some children of its own.
每个节点都有父节点(树中更高层次的节点，该节点是它的一个子级)，可能还有自己的子节点。
The thead element has a single child node, tr, with three children - all th elements.
thead元素只有一个子节点tr，后者有三个子节点—所有th元素。
The thead element has a single child node, tr, with three children - all th elements.
thead元素只有一个子节点tr，后者有三个子节点—所有th元素。
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