body{height:100%}和html,body{height:100%}有什么区别？为什么html要设置height:100%呢，html不就是整个窗口吗？

body{height:100%} and html, body {height:100%} have a crucial difference related to how the browser calculates the height of the body element. The html element represents the root of the document, while the body element contains the visible content.

Here's the breakdown:

• body { height: 100% }: This sets the body's height to 100% of its parent's height. If the parent (html) doesn't have an explicitly defined height, the body will not take up the full viewport height. It might only be as tall as its content. This is because the html element, by default, doesn't necessarily have a height of 100% of the viewport.

• html, body { height: 100% }: This sets both the html and body elements to 100% height. Crucially, setting the html element's height to 100% makes it fill the entire viewport height. Then, the body, having its height set to 100% of its parent (html), will also fill the entire viewport height. This is the key to making the body occupy the full browser window.

To your question, "Isn't html the entire window already?", the answer is subtly no. The html element contains the entire window's content, but its height isn't automatically the viewport height unless explicitly set. It adjusts to the content within it unless told otherwise.

Why is this important?

This is crucial for layouts where you want elements within the body to have percentage-based heights. If the body doesn't have a defined height relative to the viewport, percentage heights for child elements won't work as expected. They'll be calculated relative to the body's height, which might be smaller than the viewport.

In summary:

To ensure the body element takes up the full viewport height, you must set both the html and body elements' height to 100%. Setting only the body's height to 100% is insufficient if the html element's height isn't also explicitly set.