<html> vs. <body> element in css_elementin-优快云博客

Summary

The html and body elements are distinct block-level entities, in a parent/child relationship.
The html element's height and width are controlled by the browser window.
It is the html element which has (by default) overflow:auto, causing scrollbars to appear when needed.
The body element is (by default) position:static, which means that positioned children of it are positioned relative to the html element's coordinate system.
In almost all modern browsers, the built-in offset from the edge of the page is applied through a margin on the body element, not padding on the html element.

重要的一点：html 的宽和高由浏览器窗口控制

block-level elements which don't have a specific height applied to them automatically grow in height to accommodate their content. What happens when a height is applied? It depends on the value of the CSS overflow property

块级元素默认没有特别指定高度，根据包裹其中的内容自动增长高度，如果为其指定高度，接下来发生的事就要取决于 overflow 这个属性的值了。

html 元素默认 overflow 熟悉为 auto，但是没有指定它的高，为什么会出现scrollbar？

The html element is a little bit special. Its height and width are governed by the window/frame it's in. When you increase your window width, the fixed width of the html element is increased; when you make the window taller, so increases the height of the html element.

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