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，例如圖片，嵌入文檔和應用程序。例如，HTML的img標簽的內容經常被它的屬性src指定的內容替代，替代元素通常本身有大小：原有高度，原有寬度和原有比例。
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中說input可以作為替代元素的例子，但是現在的input的display是inline-block，是可以設置寬高的，所以不適合作為替代元素的例子
