css中width:auto和width:100%的区别是什么_html/css_WEB-ITnose
一、width：auto
1、块级元素默认的宽度值，意味着浏览器会自己选择一个合适的宽度值。
2、内容的宽度='margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right'
如果margin-left' + 'border-left-width' + 'padding-left' + 'padding-right' + 'border-right-width' + 'margin-right'比较大，就减小width的值，如果比较小呢，就增大width的值，使其满足上面的表达式。
二、width:100%
当width设置为100%之后，它的宽度就是父级的width，并且随着父级的width自动变化，增加子元素的padding和margin之后，它的width还是不变的，这是与设置为auto的区别。
三、width:auto和width:100%的区别
1、width:100% 并不包含margin-left margin-right的属性值，直接取其父容器的宽度加上含margin-left /margin-right的值。如果设置了margin那新的width值是容器的宽度加上margin的值。就会发现加了 margin相对应的边就会多出设置的空白。而且会多出横向滚动条因为宽度已经超出了屏幕的范围，(这条相对于父容器是body)。
2、width:auto包含margin-left/margin-right的属性值。width:auto总是占据整行，这其中margin的值已经包含其中了，如果要设置margin的值那就用一整行然后减去margin的值就得到了现在的宽度了。减去的这个值就是相应边得空白。显著的特征是这个没有横向滚动条出现也就是宽度没有增加。
3、一般width:auto使用的多，因为这样灵活，而width:100%使用比较少，因为在增加padding或者margin的时候，容易使其突破父级框，破环布局。
参考资料： css中width:auto和width:100%的区别 http://www.studyofnet.com/news/965.html
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