通过开源文档学LESS系列1

今天药忘吃喽~ 2022-08-24 14:19 164阅读 0赞

## 1.引言 ##

今天开始一个系列教程，通过一些国外开源LESS文档来学习LESS。这些开源的文档一般都是前端开发实际项目经验的总结，从中可以看出LESS的基本语法和日常使用，也可以从中管窥大牛开发者的智慧。

首先，我们来看一个开源项目Preboot，大家可以看看他的[官方网站][Link 1]，或者到[github][]中学习、下载中学习、下载本项目。作者Mark Otto，大家可以关注他的[github][github 1]。

A collection of LESS mixins and variables for writing better CSS.

[http://getpreboot.com][http_getpreboot.com]

好的，言归正传。

## 2.代码解析 ##

来吧，先看注释，注释的习惯要养成，要学会尊重别人的成果。有的时候灵感来源也可以一并写上，表示感谢。

/*!
     * Preboot v2
     *
     * Open sourced under MIT license by @mdo.
     * Some variables and mixins from Bootstrap (Apache 2 license).
     */

正文开始，首先是一些变量

//
    // Variables
    // --------------------------------------------------
    
    // Grayscale
    @black-10:                 darken(#fff, 10%);
    @black-20:                 darken(#fff, 20%);
    @black-30:                 darken(#fff, 30%);
    @black-40:                 darken(#fff, 40%);
    @black-50:                 darken(#fff, 50%);
    @black-60:                 darken(#fff, 60%);
    @black-70:                 darken(#fff, 70%);
    @black-80:                 darken(#fff, 80%);
    @black-90:                 darken(#fff, 90%);
    
    // Brand colors
    @brand-primary:           #428bca;
    @brand-success:           #5cb85c;
    @brand-warning:           #f0ad4e;
    @brand-danger:            #d9534f;
    @brand-info:              #5bc0de;
    
    // Scaffolding
    @body-background:         #fff;
    @text-color:              @black-50;
    
    // Links
    @link-color:              @brand-primary;
    @link-color-hover:        darken(@link-color, 15%);
    
    // Typography
    @font-family-sans-serif:  "Helvetica Neue", Helvetica, Arial, sans-serif;
    @font-family-serif:       Georgia, "Times New Roman", Times, serif;
    @font-family-monospace:   Monaco, Menlo, Consolas, "Courier New", monospace;
    @font-family-base:        @font-family-sans-serif;
    
    @font-size-base:          14px;
    @font-size-large:         @font-size-base * 1.25; // ~18px
    @font-size-small:         @font-size-base * 0.85; // ~12px
    @font-size-mini:          @font-size-base * 0.75; // ~11px
    
    @line-height-base:        1.4;
    
    @headings-font-family:    inherit; // empty to use BS default, @font-family-base
    @headings-font-weight:    500;
    
    // Forms
    @input-color-placeholder: lighten(@text-color, 25%);
    
    // Grid
    // Used with the grid mixins below
    @grid-columns:          12;
    @grid-column-padding:   15px; // Left and right inner padding
    @grid-float-breakpoint: 768px;

这里区块的注释的方式可以借鉴，有的时候也可以给区块加上标号，形成一个目录的方式。例如

//
    // index
    //-----------------------------------------
    //  1.Variables
    //  2.Grid system
    //  3.Mixins: vendor prefixes
    
    
    //
    // 1.Variables
    // --------------------------------------------------

全局变量的方式也可以借鉴，进行网站开发的时候可以将这部分单独剥离出来，便于日后的更新与维护。

将品牌颜色和灰度颜色作为全局变量，具有很好的可读性和可维护性。这里给大家推荐一个品牌颜色设计的工具-[BrandColors][]，大家可以去围观。

链接、字体和网格的一些设置也可以作为全局变量。

好的，这里我们来学点LESS的基础，变量和注释。

### 2.1变量 ###

变量可能是大部分爱上CSS预处理器的原因，也是诸如LESS、SASS中使用最普遍，争议最少的。

变量的定义和使用很简单，例如

@font-size-base:          14px;

变量的值可以是一个具体的数值，也可以是一个运算表达式，可以调用一个函数，甚至可以调用一个新的变量，来看看示例。

//变量的值可以是值和表达式
    @font-size-base:          14px;
    @font-size-large:         @font-size-base * 1.25; // ~18px
    //变量的值也可以调用另一变量或者函数
    @brand-primary:           #428bca;
    @link-color:              @brand-primary;
    @link-color-hover:        darken(@link-color, 15%);

### 2.2注释 ###

LESS支持两种注释方式，一种是CSS的注释，也即块注释，另一种是类似js里的行注释，这种双斜线的注释会在编译成CSS的时候被过滤掉。

好的，接着来看preboot文档

//
    // Grid system
    // --------------------------------------------------
    
    // Grid
    .make-row() {
      // Negative margin the row out to align the content of columns
      margin-left: -@grid-column-padding;
      margin-right: -@grid-column-padding;
      // Then clear the floated columns
      .clearfix();
    }
    .make-column(@columns) {
      @media (min-width: @grid-float-breakpoint) {
        float: left;
        // Calculate width based on number of columns available
        width: percentage(@columns / @grid-columns);
      }
      // Prevent columns from collapsing when empty
      min-height: 1px;
      // Set inner padding as gutters instead of margin
      padding-left: @grid-column-padding;
      padding-right: @grid-column-padding;
      // Proper box-model (padding doesn't add to width)
      .box-sizing(border-box);
    }
    .make-column-offset(@columns) {
      @media (min-width: @grid-float-breakpoint) {
        margin-left: percentage(@columns / @grid-columns);
      }
    }

这里是实现网格布局的LESS mixins。

### 2.3 混合(Mixin) ###

在 LESS 中我们可以定义一些通用的属性集为一个 class，然后在另一个 class 中去调用这些属性,例如

//定义Mixin
    .border{
    	border:2px solid #f00;
    }
    //使用Mixin
    someDiv{
    	width:200px;
    	height:200px;
    	.border;
    }

又如清除浮动的代码经常用到，我们就可以弄成一个Mixin，然后在需要的时候使用，例如

//清除浮动
    .clearfix() {
      *zoom: 1;
      &:before,
      &:after {
        content: " "; // 1
        display: table; // 2
        // Fixes Opera/contenteditable bug:
        // http://nicolasgallagher.com/micro-clearfix-hack/#comment-36952
        line-height: 0;
      }
      &:after {
        clear: both;
      }
    }
    // Grid
    .make-row() {
      // Negative margin the row out to align the content of columns
      margin-left: -@grid-column-padding;
      margin-right: -@grid-column-padding;
      // Then clear the floated columns,使用“清除浮动”的混合
      .clearfix();
    }

Mixin可以像上面那样不带参数，也可以像其他语言的函数那样接受参数，例如

.make-column(@columns) {
      @media (min-width: @grid-float-breakpoint) {
        float: left;
        // Calculate width based on number of columns available
        width: percentage(@columns / @grid-columns);
      }
      // Prevent columns from collapsing when empty
      min-height: 1px;
      // Set inner padding as gutters instead of margin
      padding-left: @grid-column-padding;
      padding-right: @grid-column-padding;
      // Proper box-model (padding doesn't add to width)
      .box-sizing(border-box);
    }

我们来继续preboot文档解析，LESS等预处理的一个重要用处就是处理vendor prefixes。

//
    // Mixins: vendor prefixes
    // --------------------------------------------------
    
    // Box sizing
    .box-sizing(@box-model) {
      -webkit-box-sizing: @box-model; // Safari <= 5
         -moz-box-sizing: @box-model; // Firefox <= 19
              box-sizing: @box-model;
    }
    
    // Single side border-radius
    .border-top-radius(@radius) {
      border-top-right-radius: @radius;
       border-top-left-radius: @radius;
    }
    .border-right-radius(@radius) {
      border-bottom-right-radius: @radius;
         border-top-right-radius: @radius;
    }
    .border-bottom-radius(@radius) {
      border-bottom-right-radius: @radius;
       border-bottom-left-radius: @radius;
    }
    .border-left-radius(@radius) {
      border-bottom-left-radius: @radius;
         border-top-left-radius: @radius;
    }
    
    // Drop shadows
    .box-shadow(@shadow) {
      -webkit-box-shadow: @shadow; // iOS <4.3 & Android <4.1
              box-shadow: @shadow;
    }
    
    // Transitions
    .transition(@transition) {
      -webkit-transition: @transition;
         -moz-transition: @transition;
           -o-transition: @transition;
              transition: @transition;
    }
    .transition-delay(@transition-delay) {
      -webkit-transition-delay: @transition-delay;
         -moz-transition-delay: @transition-delay;
           -o-transition-delay: @transition-delay;
              transition-delay: @transition-delay;
    }
    .transition-duration(@transition-duration) {
      -webkit-transition-duration: @transition-duration;
         -moz-transition-duration: @transition-duration;
           -o-transition-duration: @transition-duration;
              transition-duration: @transition-duration;
    }
    
    // Transformations
    .rotate(@degrees) {
      -webkit-transform: rotate(@degrees);
         -moz-transform: rotate(@degrees);
          -ms-transform: rotate(@degrees);
           -o-transform: rotate(@degrees);
              transform: rotate(@degrees);
    }
    .scale(@ratio) {
      -webkit-transform: scale(@ratio);
         -moz-transform: scale(@ratio);
          -ms-transform: scale(@ratio);
           -o-transform: scale(@ratio);
              transform: scale(@ratio);
    }
    .translate(@x, @y) {
      -webkit-transform: translate(@x, @y);
         -moz-transform: translate(@x, @y);
          -ms-transform: translate(@x, @y);
           -o-transform: translate(@x, @y);
              transform: translate(@x, @y);
    }
    .skew(@x, @y) {
      -webkit-transform: skew(@x, @y);
         -moz-transform: skew(@x, @y);
          -ms-transform: skewX(@x) skewY(@y); // See https://github.com/twitter/bootstrap/issues/4885
           -o-transform: skew(@x, @y);
              transform: skew(@x, @y);
      -webkit-backface-visibility: hidden; // See https://github.com/twitter/bootstrap/issues/5319
    }
    .translate3d(@x, @y, @z) {
      -webkit-transform: translate3d(@x, @y, @z);
         -moz-transform: translate3d(@x, @y, @z);
           -o-transform: translate3d(@x, @y, @z);
              transform: translate3d(@x, @y, @z);
    }
    
    // Backface visibility
    //
    // Prevent browsers from flickering when using CSS 3D transforms.
    // Default value is `visible`, but can be changed to `hidden
    // See git pull https://github.com/dannykeane/bootstrap.git backface-visibility for examples
    .backface-visibility(@visibility){
      -webkit-backface-visibility: @visibility;
         -moz-backface-visibility: @visibility;
              backface-visibility: @visibility;
    }
    
    // User select
    //
    // For selecting text on the page
    .user-select(@select) {
      -webkit-user-select: @select;
         -moz-user-select: @select;
          -ms-user-select: @select;
           -o-user-select: @select;
              user-select: @select;
    }
    
    // Opacity
    .opacity(@opacity) {
      opacity: @opacity;
      @opacity-ie: @opacity * 100;
      filter: ~"alpha(opacity=@{opacity-ie})"; // IE8
    }
    
    // Placeholder text
    .placeholder(@color: @input-color-placeholder) {
      &:-moz-placeholder            { color: @color; } // Firefox 4-18
      &::-moz-placeholder           { color: @color; } // Firefox 19+
      &:-ms-input-placeholder       { color: @color; } // Internet Explorer 10+
      &::-webkit-input-placeholder  { color: @color; } // Safari and Chrome
    }
    
    // Resize anything
    .resizable(@direction) {
      resize: @direction; // Options: horizontal, vertical, both
      overflow: auto; // Safari fix
    }
    
    // CSS3 Content Columns
    .content-columns(@width, @count, @gap) {
      -webkit-column-width: @width;
         -moz-column-width: @width;
              column-width: @width;
      -webkit-column-count: @count;
         -moz-column-count: @count;
              column-count: @count;
      -webkit-column-gap: @gap;
         -moz-column-gap: @gap;
              column-gap: @gap;
    }
    
    // Optional hyphenation
    .hyphens(@mode: auto) {
      word-wrap: break-word;
      -webkit-hyphens: @mode;
         -moz-hyphens: @mode;
          -ms-hyphens: @mode;
           -o-hyphens: @mode;
              hyphens: @mode;
    }
    
    // Gradients
    #gradient {
      .horizontal(@startColor: #555, @endColor: #333) {
        background-color: @endColor;
        background-image: -moz-linear-gradient(left, @startColor, @endColor); // FF 3.6+
        background-image: -webkit-gradient(linear, 0 0, 100% 0, from(@startColor), to(@endColor)); // Safari 4+, Chrome 2+
        background-image: -webkit-linear-gradient(left, @startColor, @endColor); // Safari 5.1+, Chrome 10+
        background-image: -o-linear-gradient(left, @startColor, @endColor); // Opera 11.10
        background-image: linear-gradient(to right, @startColor, @endColor); // Standard, IE10
        background-repeat: repeat-x;
        filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=1)",argb(@startColor),argb(@endColor))); // IE9 and down
      }
      .vertical(@startColor: #555, @endColor: #333) {
        background-color: @endColor;
        background-image: -moz-linear-gradient(top, @startColor, @endColor); // FF 3.6+
        background-image: -webkit-gradient(linear, 0 0, 0 100%, from(@startColor), to(@endColor)); // Safari 4+, Chrome 2+
        background-image: -webkit-linear-gradient(top, @startColor, @endColor); // Safari 5.1+, Chrome 10+
        background-image: -o-linear-gradient(top, @startColor, @endColor); // Opera 11.10
        background-image: linear-gradient(to bottom, @startColor, @endColor); // Standard, IE10
        background-repeat: repeat-x;
        filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=0)",argb(@startColor),argb(@endColor))); // IE9 and down
      }
      .directional(@startColor: #555, @endColor: #333, @deg: 45deg) {
        background-color: @endColor;
        background-repeat: repeat-x;
        background-image: -moz-linear-gradient(@deg, @startColor, @endColor); // FF 3.6+
        background-image: -webkit-linear-gradient(@deg, @startColor, @endColor); // Safari 5.1+, Chrome 10+
        background-image: -o-linear-gradient(@deg, @startColor, @endColor); // Opera 11.10
        background-image: linear-gradient(@deg, @startColor, @endColor); // Standard, IE10
      }
      .horizontal-three-colors(@startColor: #00b3ee, @midColor: #7a43b6, @colorStop: 50%, @endColor: #c3325f) {
        background-color: mix(@midColor, @endColor, 80%);
        background-image: -webkit-gradient(left, linear, 0 0, 0 100%, from(@startColor), color-stop(@colorStop, @midColor), to(@endColor));
        background-image: -webkit-linear-gradient(left, @startColor, @midColor @colorStop, @endColor);
        background-image: -moz-linear-gradient(left, @startColor, @midColor @colorStop, @endColor);
        background-image: -o-linear-gradient(left, @startColor, @midColor @colorStop, @endColor);
        background-image: linear-gradient(to right, @startColor, @midColor @colorStop, @endColor);
        background-repeat: no-repeat;
        filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=0)",argb(@startColor),argb(@endColor))); // IE9 and down, gets no color-stop at all for proper fallback
      }
      .vertical-three-colors(@startColor: #00b3ee, @midColor: #7a43b6, @colorStop: 50%, @endColor: #c3325f) {
        background-color: mix(@midColor, @endColor, 80%);
        background-image: -webkit-gradient(linear, 0 0, 0 100%, from(@startColor), color-stop(@colorStop, @midColor), to(@endColor));
        background-image: -webkit-linear-gradient(@startColor, @midColor @colorStop, @endColor);
        background-image: -moz-linear-gradient(top, @startColor, @midColor @colorStop, @endColor);
        background-image: -o-linear-gradient(@startColor, @midColor @colorStop, @endColor);
        background-image: linear-gradient(@startColor, @midColor @colorStop, @endColor);
        background-repeat: no-repeat;
        filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=0)",argb(@startColor),argb(@endColor))); // IE9 and down, gets no color-stop at all for proper fallback
      }
      .radial(@innerColor: #555, @outerColor: #333) {
        background-color: @outerColor;
        background-image: -webkit-gradient(radial, center center, 0, center center, 460, from(@innerColor), to(@outerColor));
        background-image: -webkit-radial-gradient(circle, @innerColor, @outerColor);
        background-image: -moz-radial-gradient(circle, @innerColor, @outerColor);
        background-image: -o-radial-gradient(circle, @innerColor, @outerColor);
        background-repeat: no-repeat;
      }
      .striped(@color: #555, @angle: 45deg) {
        background-color: @color;
        background-image: -webkit-gradient(linear, 0 100%, 100% 0, color-stop(.25, rgba(255,255,255,.15)), color-stop(.25, transparent), color-stop(.5, transparent), color-stop(.5, rgba(255,255,255,.15)), color-stop(.75, rgba(255,255,255,.15)), color-stop(.75, transparent), to(transparent));
        background-image: -webkit-linear-gradient(@angle, rgba(255,255,255,.15) 25%, transparent 25%, transparent 50%, rgba(255,255,255,.15) 50%, rgba(255,255,255,.15) 75%, transparent 75%, transparent);
        background-image: -moz-linear-gradient(@angle, rgba(255,255,255,.15) 25%, transparent 25%, transparent 50%, rgba(255,255,255,.15) 50%, rgba(255,255,255,.15) 75%, transparent 75%, transparent);
        background-image: -o-linear-gradient(@angle, rgba(255,255,255,.15) 25%, transparent 25%, transparent 50%, rgba(255,255,255,.15) 50%, rgba(255,255,255,.15) 75%, transparent 75%, transparent);
        background-image: linear-gradient(@angle, rgba(255,255,255,.15) 25%, transparent 25%, transparent 50%, rgba(255,255,255,.15) 50%, rgba(255,255,255,.15) 75%, transparent 75%, transparent);
      }
    }
    
    // Reset filters for IE
    //
    // Useful for when you want to remove a gradient from an element.
    .reset-filter() {
      filter: e(%("progid:DXImageTransform.Microsoft.gradient(enabled = false)"));
    }

首先来说说什么是vendor prefixes。大家知道，CSS3的标准制定是远超于浏览器的发展，不同浏览器对CSS3的属性实现方式，实现程度不一样。在CSS3属性的浏览器实验支持阶段，厂商在标准CSS属性前加上厂商前缀加以区分。

*  Android: `-webkit-`
 *  Chrome: `-webkit-`
 *  Firefox: `-moz-`
 *  Internet Explorer: `-ms-`
 *  iOS: `-webkit-`
 *  Opera: `-o-`
 *  Safari: `-webkit-`

这就给CSS的开发者加大了难度，所幸现在火狐基本上支持了标准属性，去掉了前缀。这些前缀最终肯定会去掉，考虑到代码未来兼容性，建议大家在开发时统一处理前缀问题，可以使用诸如prefix free等工具，或者使用相关的LESS mixin集。关于vender prefixs大家可以参考以下几篇文章。

1. [CSS Vendor Prefixes-What Are They and Why You Should Use Them][]

2. [How To Deal With Vendor Prefixes][]

### 2.4 Mixin的参数 ###

LESS的混合可以不带参数，可以带一个或多个参数，参数可以有默认值，看看例子

// 带一个参数
    .box-sizing(@box-model) {
      -webkit-box-sizing: @box-model; // Safari <= 5
         -moz-box-sizing: @box-model; // Firefox <= 19
              box-sizing: @box-model;
    }
    // 带多个参数，之间用逗号隔开
    .translate3d(@x, @y, @z) {
      -webkit-transform: translate3d(@x, @y, @z);
         -moz-transform: translate3d(@x, @y, @z);
           -o-transform: translate3d(@x, @y, @z);
              transform: translate3d(@x, @y, @z);
    }
    //有默认值的参数
    .opacity(@opacity:.5) {
      opacity: @opacity;
      @opacity-ie: @opacity * 100;
      filter: ~"alpha(opacity=@{opacity-ie})"; // IE8
    }
    //参数个数不固定
    .box-shadow(...) {
        -webkit-box-shadow: @arguments;
           -moz-box-shadow: @arguments;
                box-shadow: @arguments;
    }

LESS 提供了通过参数值控制 mixin 行为的功能，看看下面的例子。

.mixin (@s, @color) { ... }
    
    .class {
        .mixin(@switch, #888);
    }

如果要根据 @switch 的值控制 .mixin 行为，只需按照下面的方法定义 .mixin：

.mixin (dark, @color) {
        color: darken(@color, 10%);
    }
    .mixin (light, @color) {
        color: lighten(@color, 10%);
    }
    .mixin (@_, @color) {
        display: block;
    }

这样调用：

@switch: light;
    
    .class {
        .mixin(@switch, #888);
    }

就会的输出结果是：

.class {
        color: #a2a2a2;
        display: block;
    }

或者也可以用关键词 when，引入一个 guard 条件实现。上面的案例改造为

.mixin(@s, @color) when (@s = dark) {
    	color: darken(@color, 10%);
    }
    .mixin(@s, @color) when (@s = light) {
    	color: lighten(@color, 10%);
    }
    .mixin(@s, @color){
    	display: block;
    }

好的，接着来看preboot，他还分享了一些有用的混合。

//
    // Mixins: utilities
    // --------------------------------------------------
    
    // Clearfix
    //
    // Source: http://nicolasgallagher.com/micro-clearfix-hack/
    //
    // For modern browsers
    // 1. The space content is one way to avoid an Opera bug when the
    //    contenteditable attribute is included anywhere else in the document.
    //    Otherwise it causes space to appear at the top and bottom of elements
    //    that are clearfixed.
    // 2. The use of `table` rather than `block` is only necessary if using
    //    `:before` to contain the top-margins of child elements.
    .clearfix() {
      *zoom: 1;
      &:before,
      &:after {
        content: " "; // 1     --注释引用
        display: table; // 2
        // Fixes Opera/contenteditable bug:
        // http://nicolasgallagher.com/micro-clearfix-hack/#comment-36952
        line-height: 0;
      }
      &:after {
        clear: both;
      }
    }
    
    // Center-align a block level element
    .center-block() {
      display: block;
      margin-left: auto;
      margin-right: auto;
    }
    
    // Sizing shortcuts
    .size(@width, @height) {
      width: @width;
      height: @height;
    }
    .square(@size) {
      .size(@size, @size);
    }
    
    // Text overflow
    //
    // Requires inline-block or block for proper styling
    .text-truncate() {
      overflow: hidden;
      text-overflow: ellipsis;
      white-space: nowrap;
    }
    
    // Retina images
    //
    // Retina background-image support with non-retina fall back
    .retina-image(@file-1x, @file-2x, @width-1x, @height-1x) {
      background-image: url("@{file-1x}");
    
      @media
      only screen and (-webkit-min-device-pixel-ratio: 2),
      only screen and (   min--moz-device-pixel-ratio: 2),
      only screen and (     -o-min-device-pixel-ratio: 2/1),
      only screen and (        min-device-pixel-ratio: 2),
      only screen and (                min-resolution: 192dpi),
      only screen and (                min-resolution: 2dppx) {
        background-image: url("@{file-2x}");
        background-size: @width-1x @height-1x;
      }
    }

这段代码里面注释引用的方式不错。

最后有个Retina images的解决方案。Retina images也是目前前端开发的一个难题，大家可以参考相关的教程

1.[走向视网膜（Retina）的Web时代][Retina_Web]

2.[image-set实现Retina屏幕下图片显示][image-set_Retina]

3.[Retina.js为Retina设备显示高清图片][Retina.js_Retina]

## 3.总结和展望 ##

暂时到这里，本教程是一个系列教程，待续。

\---------------------------------------------------------------  
前端开发whqet，关注web前端开发技术，分享网页相关资源。  
\---------------------------------------------------------------

[Link 1]: http://getpreboot.com
[github]: https://github.com/mdo/preboot
[github 1]: http://github.com/mdo
[http_getpreboot.com]: http://getpreboot.com/
[BrandColors]: http://brandcolors.net/
[CSS Vendor Prefixes-What Are They and Why You Should Use Them]: http://webdesign.about.com/od/css/a/css-vendor-prefixes.htm
[How To Deal With Vendor Prefixes]: http://css-tricks.com/how-to-deal-with-vendor-prefixes/
[Retina_Web]: http://www.w3cplus.com/css/towards-retina-web.html
[image-set_Retina]: http://www.w3cplus.com/css/safari-6-and-chrome-21-add-image-set-to-support-retina-images.html
[Retina.js_Retina]: http://zhangqi.im/webdevelopment/retina-js.html