###导语
CSS reset的作用主要就是清楚浏览器的默认样式，比如我们在Chrome里看到的User Agent Stylesheet就是Chrome浏览器的默认样式。不同的浏览器会有不同的默认样式，因此我们需要清除一些默认样式，以达到在所有浏览器里显示效果的一致。
不过这里面的学问还是很大的。首先我们只需要reset我们在项目中会使用的元素，这就涉及reset的模块化使用。第二我们只需要reset必要的属性。因为reset会加重浏览器渲染的性能开销，因此我们在css reset时必须要慎之又慎，保证最少的代码量和最佳的效果之间的平衡。
###确定需要reset的元素
需要reset的元素有以下几类
1.块级元素
块级元素所需要的重置主要是针对边距、字体的重置。我们常用的元素有body,html,h1,h2,h3,h4,h5,h6,p,ul,li,ol,video等。
2.表单元素
块级元素所需要的重置除了边距之外，还有边框，outline等表单元素特有的属性的重置。我们常用的元素有input,button,textarea,fieldset等。
3.带特殊样式的元素
带特殊样式的元素包括带list-style的ul,ol，带text-decoration的a。
4.HTML5元素
在不支持HTML5标签的浏览器，如IE8及以下的浏览器中，在使用html5shiv这样的polyfill之后，我们需要手动给这些元素加上display:block;来设置默认的display属性。不过目前html5shiv已经包括了默认的样式，所以我们可以不用考虑HTML5元素的display属性reset。
5.images
image标签在IE8以以下版本的浏览器中默认会存在边框，所以我们需要重置border:0;。
###几个流行的reset.css
1.Eric Meyer’s “Reset CSS” 2.0
/* http://meyerweb.com/eric/tools/css/reset/
v2.0 | 20110126
License: none (public domain)
*/
html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed,
figure, figcaption, footer, header, hgroup,
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
margin: 0;
padding: 0;
border: 0;
font-size: 100%;
font: inherit;
vertical-align: baseline;
}
/* HTML5 display-role reset for older browsers */
article, aside, details, figcaption, figure,
footer, header, hgroup, menu, nav, section {
display: block;
}
body {
line-height: 1;
}
ol, ul {
list-style: none;
}
blockquote, q {
quotes: none;
}
blockquote:before, blockquote:after,
q:before, q:after {
content: '';
content: none;
}
table {
border-collapse: collapse;
border-spacing: 0;
}
评价：这是最早流行的reset.css。本文中的示例也参考了这个著名的reset.css。关于blockquote和table相关的reset可以参考这个文件。
/*
YUI 3.18.1 (build f7e7bcb)
Copyright 2014 Yahoo! Inc. All rights reserved.
Licensed under the BSD License.
http://yuilibrary.com/license/
*/
html {
color: #000;
background: #FFF
}
body,
div,
dl,
dt,
dd,
ul,
ol,
li,
h1,
h2,
h3,
h4,
h5,
h6,
pre,
code,
form,
fieldset,
legend,
input,
textarea,
p,
blockquote,
th,
td {
margin: 0;
padding: 0
}
table {
border-collapse: collapse;
border-spacing: 0
}
fieldset,
img {
border: 0
}
address,
caption,
cite,
code,
dfn,
em,
strong,
th,
var {
font-style: normal;
font-weight: normal
}
ol,
ul {
list-style: none
}
caption,
th {
text-align: left
}
h1,
h2,
h3,
h4,
h5,
h6 {
font-size: 100%;
font-weight: normal
}
q:before,
q:after {
content: ''
}
abbr,
acronym {
border: 0;
font-variant: normal
}
sup {
vertical-align: text-top
}
sub {
vertical-align: text-bottom
}
input,
textarea,
select {
font-family: inherit;
font-size: inherit;
font-weight: inherit;
*font-size: 100%
}
legend {
color: #000
}
评价：比起Eric Meyer的reset来说要更全面一些。加入了对html元素颜色的重置。在块级元素的重置上只设置了margin和padding。对行高并没有进行重置。
/*! normalize.css v3.0.2 | MIT License | git.io/normalize */
/**
* 1. Set default font family to sans-serif.
* 2. Prevent iOS text size adjust after orientation change, without disabling
* user zoom.
*/
html {
font-family: sans-serif; /* 1 */
-ms-text-size-adjust: 100%; /* 2 */
-webkit-text-size-adjust: 100%; /* 2 */
}
/**
* Remove default margin.
*/
body {
margin: 0;
}
/* HTML5 display definitions
========================================================================== */
/**
* Correct `block` display not defined for any HTML5 element in IE 8/9.
* Correct `block` display not defined for `details` or `summary` in IE 10/11
* and Firefox.
* Correct `block` display not defined for `main` in IE 11.
*/
article,
aside,
details,
figcaption,
figure,
footer,
header,
hgroup,
main,
menu,
nav,
section,
summary {
display: block;
}
/**
* 1. Correct `inline-block` display not defined in IE 8/9.
* 2. Normalize vertical alignment of `progress` in Chrome, Firefox, and Opera.
*/
audio,
canvas,
progress,
video {
display: inline-block; /* 1 */
vertical-align: baseline; /* 2 */
}
/**
* Prevent modern browsers from displaying `audio` without controls.
* Remove excess height in iOS 5 devices.
*/
audio:not([controls]) {
display: none;
height: 0;
}
/**
* Address `[hidden]` styling not present in IE 8/9/10.
* Hide the `template` element in IE 8/9/11, Safari, and Firefox < 22.
*/
[hidden],
template {
display: none;
}
/* Links
========================================================================== */
/**
* Remove the gray background color from active links in IE 10.
*/
a {
background-color: transparent;
}
/**
* Improve readability when focused and also mouse hovered in all browsers.
*/
a:active,
a:hover {
outline: 0;
}
/* Text-level semantics
========================================================================== */
/**
* Address styling not present in IE 8/9/10/11, Safari, and Chrome.
*/
abbr[title] {
border-bottom: 1px dotted;
}
/**
* Address style set to `bolder` in Firefox 4+, Safari, and Chrome.
*/
b,
strong {
font-weight: bold;
}
/**
* Address styling not present in Safari and Chrome.
*/
dfn {
font-style: italic;
}
/**
* Address variable `h1` font-size and margin within `section` and `article`
* contexts in Firefox 4+, Safari, and Chrome.
*/
h1 {
font-size: 2em;
margin: 0.67em 0;
}
/**
* Address styling not present in IE 8/9.
*/
mark {
background: #ff0;
color: #000;
}
/**
* Address inconsistent and variable font size in all browsers.
*/
small {
font-size: 80%;
}
/**
* Prevent `sub` and `sup` affecting `line-height` in all browsers.
*/
sub,
sup {
font-size: 75%;
line-height: 0;
position: relative;
vertical-align: baseline;
}
sup {
top: -0.5em;
}
sub {
bottom: -0.25em;
}
/* Embedded content
========================================================================== */
/**
* Remove border when inside `a` element in IE 8/9/10.
*/
img {
border: 0;
}
/**
* Correct overflow not hidden in IE 9/10/11.
*/
svg:not(:root) {
overflow: hidden;
}
/* Grouping content
========================================================================== */
/**
* Address margin not present in IE 8/9 and Safari.
*/
figure {
margin: 1em 40px;
}
/**
* Address differences between Firefox and other browsers.
*/
hr {
-moz-box-sizing: content-box;
box-sizing: content-box;
height: 0;
}
/**
* Contain overflow in all browsers.
*/
pre {
overflow: auto;
}
/**
* Address odd `em`-unit font size rendering in all browsers.
*/
code,
kbd,
pre,
samp {
font-family: monospace, monospace;
font-size: 1em;
}
/* Forms
========================================================================== */
/**
* Known limitation: by default, Chrome and Safari on OS X allow very limited
* styling of `select`, unless a `border` property is set.
*/
/**
* 1. Correct color not being inherited.
* Known issue: affects color of disabled elements.
* 2. Correct font properties not being inherited.
* 3. Address margins set differently in Firefox 4+, Safari, and Chrome.
*/
button,
input,
optgroup,
select,
textarea {
color: inherit; /* 1 */
font: inherit; /* 2 */
margin: 0; /* 3 */
}
/**
* Address `overflow` set to `hidden` in IE 8/9/10/11.
*/
button {
overflow: visible;
}
/**
* Address inconsistent `text-transform` inheritance for `button` and `select`.
* All other form control elements do not inherit `text-transform` values.
* Correct `button` style inheritance in Firefox, IE 8/9/10/11, and Opera.
* Correct `select` style inheritance in Firefox.
*/
button,
select {
text-transform: none;
}
/**
* 1. Avoid the WebKit bug in Android 4.0.* where (2) destroys native `audio`
* and `video` controls.
* 2. Correct inability to style clickable `input` types in iOS.
* 3. Improve usability and consistency of cursor style between image-type
* `input` and others.
*/
button,
html input[type="button"], /* 1 */
input[type="reset"],
input[type="submit"] {
-webkit-appearance: button; /* 2 */
cursor: pointer; /* 3 */
}
/**
* Re-set default cursor for disabled elements.
*/
button[disabled],
html input[disabled] {
cursor: default;
}
/**
* Remove inner padding and border in Firefox 4+.
*/
button::-moz-focus-inner,
input::-moz-focus-inner {
border: 0;
padding: 0;
}
/**
* Address Firefox 4+ setting `line-height` on `input` using `!important` in
* the UA stylesheet.
*/
input {
line-height: normal;
}
/**
* It's recommended that you don't attempt to style these elements.
* Firefox's implementation doesn't respect box-sizing, padding, or width.
*
* 1. Address box sizing set to `content-box` in IE 8/9/10.
* 2. Remove excess padding in IE 8/9/10.
*/
input[type="checkbox"],
input[type="radio"] {
box-sizing: border-box; /* 1 */
padding: 0; /* 2 */
}
/**
* Fix the cursor style for Chrome's increment/decrement buttons. For certain
* `font-size` values of the `input`, it causes the cursor style of the
* decrement button to change from `default` to `text`.
*/
input[type="number"]::-webkit-inner-spin-button,
input[type="number"]::-webkit-outer-spin-button {
height: auto;
}
/**
* 1. Address `appearance` set to `searchfield` in Safari and Chrome.
* 2. Address `box-sizing` set to `border-box` in Safari and Chrome
* (include `-moz` to future-proof).
*/
input[type="search"] {
-webkit-appearance: textfield; /* 1 */
-moz-box-sizing: content-box;
-webkit-box-sizing: content-box; /* 2 */
box-sizing: content-box;
}
/**
* Remove inner padding and search cancel button in Safari and Chrome on OS X.
* Safari (but not Chrome) clips the cancel button when the search input has
* padding (and `textfield` appearance).
*/
input[type="search"]::-webkit-search-cancel-button,
input[type="search"]::-webkit-search-decoration {
-webkit-appearance: none;
}
/**
* Define consistent border, margin, and padding.
*/
fieldset {
border: 1px solid #c0c0c0;
margin: 0 2px;
padding: 0.35em 0.625em 0.75em;
}
/**
* 1. Correct `color` not being inherited in IE 8/9/10/11.
* 2. Remove padding so people aren't caught out if they zero out fieldsets.
*/
legend {
border: 0; /* 1 */
padding: 0; /* 2 */
}
/**
* Remove default vertical scrollbar in IE 8/9/10/11.
*/
textarea {
overflow: auto;
}
/**
* Don't inherit the `font-weight` (applied by a rule above).
* NOTE: the default cannot safely be changed in Chrome and Safari on OS X.
*/
optgroup {
font-weight: bold;
}
/* Tables
========================================================================== */
/**
* Remove most spacing between table cells.
*/
table {
border-collapse: collapse;
border-spacing: 0;
}
td,
th {
padding: 0;
}
评价：这是一个全面的reset.css。最大的特点就是模块化。分HTML5 display definitions,Links,Text-level semantics,Embedded content,Grouping content,Forms,Tables这几个模块来分别设置。都有详细的注释。是一份不错的学习材料。
###font-famliy的设置
在根元素处设置的font-famliy属性决定了全站所有页面的默认字体。一般我们设置font-famliy的顺序为，默认英文字体，默认中文字体，fallback中文字体。
比如:
font-famliy:"Arial","Helvetica","Microsoft YaHei","PingFang SC","STHeiti","sans-serif";
首先我们设置默认的英文字体。因为中文字体之中是带有一套英文字体的，所有我们会首先设置默认英文字体来防止被覆盖。然后我们设置windows和OS X上的默认中文字体，这里一般用英文来设置，来避免一些兼容性问题。最后我们设置sans-serif衬线字体来做一个fallback。
对于字体要求一般的情况下，这样的设置其实就满足我们的要求了。
font-famliy:"Arial","Helvetica","sans-serif";
这种设置下，系统会调用默认的无衬线中文字体。所以如果在某些有很多无衬线中文字体的操作系统，如OS X下没有特定的字体要求的话，可以将第一种设置简化为这种设置。移动端web的字体比较多，所以设置方式与上面的类似。
###一个基本的reset.css示例
/* 全局
===================================================================== */
/**
* 重置默认行高
*/
body{
line-height:1;}
html{
margin:0;
}
/* 文字语义标签
===================================================================== */
h1,h2,h3,h4,h5,h6{
marign:0;
font-size:100%;
font-weight: normal;
}
p{
margin:0;
}
/* 表单
===================================================================== */
fieldset{
border:0;
}
input,button,textarea{
border:0;
outline:0;
padding:0;
}
/**
* chrome下button默认的box-sizing属性是border-box
*/
button{
box-sizing:content-box;
}
/* 链接
===================================================================== */
a{
color:#fff;
text-decoration:none;
}
/* 嵌入内容
===================================================================== */
img{
border:0;
}
/* 列表
===================================================================== */
ol, ul {
list-style: none;
}
/* 表格
===================================================================== */
table {
border-collapse: collapse;
border-spacing: 0
}
###结语
上面列出的示例，其实如Meyer说的，只是一个开始。我们需要根据我们的具体需求来做微调，来达到最好的“性价比”。
比如众多的HTML5元素。如果你的项目中不打算使用这些元素，那么你就不用添加对这些元素的reset。之前列的三个流行reset的内容长短不一，很大程度上也是因为大家的目标不一样。所以我们应该以这些优秀的reset.css为模板，来打造自己的reset.css。