Error description with element hidden check in "Element size and scrolling"

The tutorial said:

> Geometry properties are calculated only for displayed elements.
>
> If an element (or any of its ancestors) has display:none or is not in the document, then all geometry properties are zero (or null for offsetParent).
>
> For example, offsetParent is null, and offsetWidth, offsetHeight are 0 when we created an element, but haven’t inserted it into the document yet, or it (or it’s ancestor) has display:none.
>
> We can use this to check if an element is hidden, like this:
>
> ```js
> function isHidden(elem) {
> return !elem.offsetWidth && !elem.offsetHeight;
> }
> ```

But, what if the hidden element is hidden by setting the `width` to `0`, not the `height`? 😀

從「元素大小和捲動」教學章節開始，檢視引用的幾何屬性說明和 isHidden 範例。釐清當元素寬度為零時範例的行為，並確保完成的文字準確說明隱藏元素檢查。

由索引模型根據 Issue 內容生成。