时间:2021-07-01 10:21:17 帮助过:25人阅读
.Absolute-Center { width: 50%; height: 50%; overflow: auto; margin: auto; position: absolute; top: 0; left: 0; bottom: 0; right: 0; background-color: red; }
在线演示
在普通文档流里,margin: auto; 的意思是设置元素的margin-top和margin-bottom为0。
W3.org:?If ‘margin-top’, or ‘margin-bottom’ are ‘auto’, their used value is 0.
设置了position: absolute; 的元素会变成块元素,并脱离普通文档流。而文档的其余部分照常渲染,元素像是不在原来的位置一样。
Developer.mozilla.org:?…an element that is positioned absolutely is taken out of the flow and thus takes up no space
设置了top: 0; left: 0; bottom: 0; right: 0; 样式的块元素会让浏览器为它包裹一层新的盒子,因此这个元素会填满它相对父元素的内部空间,这个相对父元素可以是是body标签,或者是一个设置了position: relative; 样式的容器。
Developer.mozilla.org:?For absolutely positioned elements, the top, right, bottom, and left properties specify offsets from the edge of the element’s containing block (what the element is positioned relative to).
给元素设置了宽高以后,浏览器会阻止元素填满所有的空间,根据margin: auto; 的要求,重新计算,并包裹一层新的盒子。
Developer.mozilla.org:?The margin of the [absolutely positioned] element is then positioned inside these offsets.
既然块元素是绝对定位的,又脱离了普通文档流,因此浏览器在包裹盒子之前会给margin-top和margin-bottom设置一个相等的值。
W3.org:?If none of the three [top, bottom, height] are ‘auto’: If both ‘margin-top’ and ‘margin-bottom’ are ‘auto’, solve the equation under the extra constraint that the two margins get equal values.?AKA: center the block vertically
使用“完全居中”,有意遵照了标准margin: auto; 样式渲染的规定,所以应当在与标准兼容的各种浏览器中起作用。
优点:
同时注意:
.negative-margin { width: 300px; height: 200px; padding: 20px; position: absolute; top: 50%; left: 50%; margin-left: -170px; /* (width + padding)/2 */ margin-top: -120px; /* (height + padding)/2 */}
在线演示
.transform { width: 50%; margin: auto; position: absolute; top: 50%; left: 50%; -webkit-transform: translate(-50%,-50%); -ms-transform: translate(-50%,-50%); transform: translate(-50%,-50%);}
在线演示
HTML:
CSS:
.Center-Container.is-Inline { text-align: center; overflow: auto;} .Center-Container.is-Inline:after,.is-Inline .Center-Block { display: inline-block; vertical-align: middle;} .Center-Container.is-Inline:after { content: ''; height: 100%; margin-left: -0.25em; /* To offset spacing. May vary by font */} .is-Inline .Center-Block { max-width: 99%; /* Prevents issues with long content causes the content block to be pushed to the top */ /* max-width: calc(100% - 0.25em) /* Only for IE9+ */ }
在线演示
.Center-Container.is-Flexbox { display: -webkit-box; display: -moz-box; display: -ms-flexbox; display: -webkit-flex; display: flex; -webkit-box-align: center; -moz-box-align: center; -ms-flex-align: center; -webkit-align-items: center; align-items: center; -webkit-box-pack: center; -moz-box-pack: center; -ms-flex-pack: center; -webkit-justify-content: center; justify-content: center;}
在线演示
优点:
同时注意:
HTML:
CSS:
.Center-Container.is-Table { display: table; }.is-Table .Table-Cell { display: table-cell; vertical-align: middle;}.is-Table .Center-Block { width: 50%; margin: 0 auto;}
在线演示
参考出处: