important > 内联(1,0,0,0) > id(1,0,0) > class(1,0) > element(1) > *通配符

方式一：在head里用link标签(推荐使用)
<link rel="stylesheet" href="xxx.css">方式二：在head里用style标签
<style> some css </style>方式三：内联，在标签上用style
<div style=" some css "></div>方式四：@import
@import url("xxx.css");link和@import的区别：
1.兼容性：@import在IE5+支持，link全部支持
2.DOM样式操作：link（可被js改变），@import（不可被js改变）
3.加载顺序：@import必须放在最顶部，和link混用时破坏并行加载，link阻断@import，多个@import不同于预期加载顺序

/* IE 6 and below */
* html .selector  {}
.suckyie6.selector {} /* .suckyie6 can be any unused class *//* IE 7 and below */
.selector, {}/* IE 7 */
*:first-child+html .selector {}
.selector, x:-IE7 {}
*+html .selector {}/* Everything but IE 6 */
html > body .selector {}/* Everything but IE 6/7 */
html > /**/ body .selector {}
head ~ /* */ body .selector {}/* Everything but IE 6/7/8 */
:root *> .selector {}
body:last-child .selector {}
body:nth-of-type(1) .selector {}
body:first-of-type .selector {}

/* IE 6 */
.selector { _color: blue; }
.selector { -color: blue; }/* IE 6/7 - acts as an !important */
.selector { color: blue !ie; }
/* string after ! can be anything *//* IE 6/7 - any combination of these characters: ! $ & * ( ) = % + @ , . / ` [ ] # ~ ? : < > | */
.selector { !color: blue; }
.selector { $color: blue; }
.selector { &color: blue; }
.selector { *color: blue; }
/* ... *//* IE 8/9 */
.selector { color: blue\0/; }
/* must go at the END of all rules *//* IE 8/9 */
.selector { color: blue\0/; }
/* must go at the END of all rules *//* IE 9/10 */
.selector:nth-of-type(1n) { color: blue\9; }/* IE 6/7/8/9/10 */
.selector { color: blue\9; }
.selector { color/*\**/: blue\9; }/* Everything but IE 6 */
.selector { color/**/: blue; }

/* IE 6/7 */
@media screen\9 {}/* IE 8 */
@media \0screen {}/* IE 9/10, Firefox 3.5+, Opera */
@media screen and (min-resolution: +72dpi) {}/* IE 10+ */
@media screen and (-ms-high-contrast: active), (-ms-high-contrast: none) {}/* IE 6/7/8 */
@media \0screen\,screen\9 {}/* IE 8/9/10 & Opera */
@media screen\0 {}/* IE 9/10 */
@media screen and (min-width:0\0) {}/* Everything but IE 6/7/8 */
@media screen and (min-width: 400px) {}

/* IE 6 */
(checkIE = document.createElement("b")).innerHTML = "<!--[if IE 6]><i></i><![endif]-->";
var isIE = checkIE.getElementsByTagName("i").length == 1;/* IE 7 */
(checkIE = document.createElement("b")).innerHTML = "<!--[if IE 7]><i></i><![endif]-->";
var isIE = checkIE.getElementsByTagName("i").length == 1;
navigator.appVersion.indexOf("MSIE 7.")!=-1/* IE <= 8 */
var isIE = '\v'=='v';/* IE 8 */
(checkIE = document.createElement("b")).innerHTML = "<!--[if IE 8]><i></i><![endif]-->";
var isIE = checkIE.getElementsByTagName("i").length == 1;/* IE 9 */
(checkIE = document.createElement("b")).innerHTML = "<!--[if IE 9]><i></i><![endif]-->";
var isIE = checkIE.getElementsByTagName("i").length == 1;/* IE 10 */
var isIE = eval("/*@cc_on!@*/false") && document.documentMode === 10;/* IE 10 */
var isIE = document.body.style.msTouchAction != undefined;

选择器Hack/* Firefox 1.5 */
body:empty .selector {}/* Firefox 2+ */
.selector, x:-moz-any-link {}/* Firefox 3+ */
.selector, x:-moz-any-link; x:default {}/* Firefox 3.5+ */
body:not(:-moz-handler-blocked) .selector {}媒体查询 Hack/* Firefox 3.5+, IE 9/10, Opera */
@media screen and (min-resolution: +72dpi) {}/* Firefox 3.6+ */
@media screen and (-moz-images-in-menus:0) {}/* Firefox 4+ */
@media screen and (min--moz-device-pixel-ratio:0) {}Javascript Hack/* Firefox */
var isFF = !!navigator.userAgent.match(/firefox/i);/* Firefox 2 - 13 */
var isFF = Boolean(window.globalStorage);/* Firefox 2/3 */
var isFF = /a/[-1]=='a';/* Firefox 3 */
var isFF = (function x(){})[-5]=='x';

选择器 Hack/* Chrome 24- and Safari 5- */
::made-up-pseudo-element, .selector {}媒体查询 Hack/* Chrome, Safari 3+ */
@media screen and (-webkit-min-device-pixel-ratio:0) {}Javascript Hack/* Chrome */
var isChrome = Boolean(window.chrome);

选择器Hack/* Safari 2/3 */
html[xmlns*=""] body:last-child .selector {}
html[xmlns*=""]:root .selector  {}/* Safari 2/3.1, Opera 9.25 */
*|html[xmlns*=""] .selector {}/* Safari 5- and Chrome 24- */
::made-up-pseudo-element, .selector {}媒体查询Hack/* Safari 3+, Chrome */
@media screen and (-webkit-min-device-pixel-ratio:0) {}Javascript Hack/* Safari */
var isSafari = /a/.__proto__=='//';

选择器Hack/* Opera 9.25, Safari 2/3.1 */
*|html[xmlns*=""] .selector {}/* Opera 9.27 and below, Safari 2 */
html:first-child .selector {}/* Opera 9.5+ */
noindex:-o-prefocus, .selector {}媒体查询Hack/* Opera 7 */
@media all and (min-width: 0px){}/* Opera 12- */
@media all and (-webkit-min-device-pixel-ratio:10000), not all and (-webkit-min-device-pixel-ratio:0) {}/* Opera, Firefox 3.5+, IE 9/10 */
@media screen and (min-resolution: +72dpi) {}/* Opera, IE 8/9/10 */
@media screen {}Javascript Hack/* Opera 9.64- */
var isOpera = /^function \(/.test([].sort);/* Opera 12- */
var isOpera = Boolean(window.opera);