onunload事件在用户离开页面时发生。
两个主要的响应时期是：页面提交之后，或在用户的焦点离开字段时立即响应。
The two primary response periods are: after a page has been submitted, and immediately after user focus leaves a field.
有良好可读性的文字不仅容易理解，容易快速阅读，也能提升网站用户继续读下去——而不是关闭页面离开的可能性。
Easy readability of text improves comprehension and reading speed as well as enhancing the likelihood that a user will continue reading instead of abandoning the web page.
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