If one principle defined the Document Object Model, it would be that information is arranged as a parent-child hierarchy. For example, the following XML.
如果存在一个定义文档对象模型的原则，它将把信息排列成父-子层次结构。
A child font size in one selector becomes a parent font size in another selector in the lower level in the hierarchy of selector sets.
其中一个选择器中的子字体大小，将成为处于选择器集合中较低层次的另一个选择器的父字体大小。
A containment hierarchy allows the access of all child elements from a parent very easily, for instance, access to all orders associated with a specific customer.
容器层次结构允许您毫不费力地访问父元素中的所有子元素，例如，允许访问与某一特定客户相关的所有订单。
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