CSS 盒模型、解决方案

W3C 标准盒模型 & IE 怪异盒模型

页面上显示的每个元素（包括内联元素）都可以看作一个盒子，即盒模型( box model )

盒模型由 4 部分组成，从内到外分别是：content padding border margin

W3C 标准盒模型一个元素的宽度（高度以此类推）应该这样计算：
一个元素的宽度 = content
盒子总宽度 = margin-left + border-left + padding-left + width + padding-right + border-right + margin-right

而IE 怪异盒模型一个元素的宽度（高度以此类推）却是这样计算的：

一个元素的宽度 =  content + padding + border
盒子总宽度 = margin-left + width + margin-right

// W3C 标准盒模型（浏览器默认）

box-sizing: content-box;

// IE 怪异盒模型

box-sizing: border-box;

当我们设置 box-sizing: border-box; 时，border 和 padding 就被包含在了宽高之内，和 IE 盒模型是一样的。

所以，为了避免你同一份 css 在不同浏览器下表现不同，最好加上：

\*, *:before, *:after {

-moz-box-sizing: border-box;

-webkit-box-sizing: border-box;

box-sizing: border-box;

}

JS 如何获取盒模型对应的宽和高
let box = document.getElementById('box')

// 只能取到内联样式的宽高
console.log('style:' + box.style.width) // 100px

// 内联样式和外联样式的宽高都能取到，但只有 IE 支持
console.log('currentStyle:' + box.currentStyle.width) // 100px

// 内联样式和外联样式的宽高都能取到，几乎所有主流浏览器都支持
console.log('getComputedStyle:' + getComputedStyle(box).width) // 100px

// 内联样式和外联样式的宽高都能取到，几乎所有主流浏览器都支持，取到的是盒子总宽度
console.log('getBoundingClientRect:' + box.getBoundingClientRect().width) // 210

转自：http://www.lovebxm.com/2017/08/27/css-box/

Every current browser supports box-sizing: border-box; unprefixed, so the need for vendor prefixes is fading. But, if you need to support older versions of Safari (< 5.1), Chrome (< 10), and Firefox (< 29), you should include the prefixes -webkit and -moz, like this:

html {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}
*, *:before, *:after {
  -webkit-box-sizing: inherit;
  -moz-box-sizing: inherit;
  box-sizing: inherit;
  }

box-sizing: border-box; is supported in the current versions of all major browsers. The less-commonly used padding-box is only supported in Firefox at the moment. There's more comprehensive information about browser support in our [box-sizing](https://css-tricks.com/almanac/properties/b/box-sizing/) almanac entry.

There are a few issues with older versions of Internet Explorer (8 and older). IE 8 doesn't recognize border-box on elements with min/max-width or min/max-height (this used to affect Firefox too, but it was fixed in 2012). IE 7 and below do not recognize box-sizing at all, but there's a polyfill that can help.

参考：https://css-tricks.com/box-sizing/