浏览器对某些选择符的支持还不是很充分，但是CSS的设计允许平稳退化；如果无法应用某个样式，页面应该仍然是合法有效的。
Browser support for some is limited, but CSS is designed for graceful degradation; a page should still be legible and usable if a style isn't applied.
HTML5的设计原则就是在不支持它的浏览器中能够平稳地退化。
HTML 5 was explicitly designed to degrade gracefully in browsers that don't support it.
在老式浏览器中显示时，允许内容平稳地退化。
Allows content to degrade gracefully when displayed in older browsers.
在老式浏览器中显示时，允许内容平稳地退化。
Allows content to degrade gracefully when displayed in older browsers.
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