For example, if rolling the mouse over a 'Buy now' link is meant to make the price, elsewhere on the page, become emphasized, JavaScript code can do this, but JavaScript needs to identify the price element, wherever it is in the markup.
例如，如果把鼠标移至"Buy now"的超链接上，它就意味价格，JavaScript代码可以做到这个，但无论它在标记中的哪里，JavaScript需要标识价格元素。