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。
HTML 5 was explicitly designed to degrade gracefully in browsers that don't support it.
HTML 5的设计原则就是在不支持它的浏览器中能够平稳地退化。
Your application should degrade gracefully if a particular sensor is not available on a specific device.
如果某个传感器无法在特定的设备上使用，您的应用程序就会适当地降级。
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