元素不会得到它的 currentStyle 属性，直到它被添加到文档，这是有道理的：直到元素被添加到文档，浏览器不能知道哪些现有的样式规则将适用于它。

An element doesn't get its currentStyle property populated until it's added to the document, which makes some sense: until the element's been added to the document, the browser cannot know which existing style rules will apply to it.