Returning a deep clone enables the runtime to support the case in which multiple custom bindings share a custom binding element.
通过返回深层克隆，运行时将能够支持多个自定义绑定共用一个自定义绑定元素的情况。
SVGBindBegin corresponds to the initial binding phase, when the custom element has been parsed but the shadow tree has not yet been built and rendered.
SVGBindBegin 对应绑定的起始阶段，即定制元素已经被解析但还没有建立和呈现为影像树的时候。
SVGBindBegin corresponds to the initial binding phase, when the custom element has been parsed but the shadow tree has not yet been built and rendered.
SVGBindBegin 对应绑定的起始阶段，即定制元素已经被解析但还没有建立和呈现为影像树的时候。
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