Replaced element
An element whose content is outside the scope of the CSS formatting model, such as an image, embedded document, or applet. For example, the content of the HTML IMG element is often replaced by the image that its "src" attribute designates. Replaced elements often have intrinsic dimensions: an intrinsic width, an intrinsic height, and an intrinsic ratio. For example, a bitmap image has an intrinsic width and an intrinsic height specified in absolute units (from which the intrinsic ratio can obviously be determined). On the other hand, other documents may not have any intrinsic dimensions (for example, a blank HTML document).
User agents may consider a replaced element to not have any intrinsic dimensions if it is believed that those dimensions could leak sensitive information to a third party. For example, if an HTML document changed intrinsic size depending on the user's bank balance, then the UA might want to act as if that resource had no intrinsic dimensions.
The content of replaced elements is not considered in the CSS rendering model.
在CSS的格式化模型之外的元素，例如图片，嵌入文档和应用程序。例如，HTML的img标签的内容经常被它的属性src指定的内容替代，替代元素通常本身有大小：原有高度，原有宽度和原有比例。
Replaced element
An element for which the CSS formatter knows only the intrinsic dimensions. In HTML, IMG, INPUT, TEXTAREA, SELECT, and OBJECT elements can be examples of replaced elements. For example, the content of the IMG element is often replaced by the image that the "src" attribute designates. CSS does not define how the intrinsic dimensions are found.
注意：在CSS1中说input可以作为替代元素的例子，但是现在的input的display是inline-block，是可以设置宽高的，所以不适合作为替代元素的例子
