前端webp图片

webp 是目前 Web 比较流行的解决方案，相对于 Jpeg/png, 基于 VP8 的压缩，有着非常不错的压缩率。

<pliga' 1,="" 'onum'="" 'kern'="" 1;="" color:="" rgb(58,="" 65,="" 69);="" font-family:="" tahoma,="" "helvetica="" neue",="" helvetica,="" arial,="" "hiragino="" sans="" gb",="" sans-serif;="" font-size:="" 16px;="" font-style:="" normal;="" font-variant-ligatures:="" font-variant-caps:="" font-weight:="" letter-spacing:="" 0.12px;="" orphans:="" 2;="" text-align:="" start;="" text-indent:="" 0px;="" text-transform:="" none;="" white-space:="" widows:="" word-spacing:="" -webkit-text-stroke-="" background-color:="" rgb(255,="" 255,="" 255);="" text-decoration-style:="" initial;="" text-decoration-color:="" initial;"="">比较基础的方法，还是检测 UA 白名单来说，毕竟这些版本都是很早就支持。

<pliga' 1,="" 'onum'="" 'kern'="" 1;="" color:="" rgb(58,="" 65,="" 69);="" font-family:="" tahoma,="" "helvetica="" neue",="" helvetica,="" arial,="" "hiragino="" sans="" gb",="" sans-serif;="" font-size:="" 16px;="" font-style:="" normal;="" font-variant-ligatures:="" font-variant-caps:="" font-weight:="" letter-spacing:="" 0.12px;="" orphans:="" 2;="" text-align:="" start;="" text-indent:="" 0px;="" text-transform:="" none;="" white-space:="" widows:="" word-spacing:="" -webkit-text-stroke-="" background-color:="" rgb(255,="" 255,="" 255);="" text-decoration-style:="" initial;="" text-decoration-color:="" initial;"="">这个方法可控性大，而且能够支持 SSR 渲染，在服务端做 UA 判断然后输出对应的图片格式。

<pliga' 1,="" 'onum'="" 'kern'="" 1;="" color:="" rgb(58,="" 65,="" 69);="" font-family:="" tahoma,="" "helvetica="" neue",="" helvetica,="" arial,="" "hiragino="" sans="" gb",="" sans-serif;="" font-size:="" 16px;="" font-style:="" normal;="" font-variant-ligatures:="" font-variant-caps:="" font-weight:="" letter-spacing:="" 0.12px;="" orphans:="" 2;="" text-align:="" start;="" text-indent:="" 0px;="" text-transform:="" none;="" white-space:="" widows:="" word-spacing:="" -webkit-text-stroke-="" background-color:="" rgb(255,="" 255,="" 255);="" text-decoration-style:="" initial;="" text-decoration-color:="" initial;"="">当然，常规的另外一种解决方式是，就是远程加载一张 webp 图片观测是否报错

function checkWebPSupport) {
  return new Promise((resolve, reject) => {
	var img = new Image();
	img.onload = function() { resolve(); };
    img.onerror = function() { reject(); };
    img.src = 'http://www.gstatic.com/webp/gallery/1.webp';
  })
}

<pliga' 1,="" 'onum'="" 'kern'="" 1;="" color:="" rgb(58,="" 65,="" 69);="" font-family:="" tahoma,="" "helvetica="" neue",="" helvetica,="" arial,="" "hiragino="" sans="" gb",="" sans-serif;="" font-size:="" 16px;="" font-style:="" normal;="" font-variant-ligatures:="" font-variant-caps:="" font-weight:="" letter-spacing:="" 0.12px;="" orphans:="" 2;="" text-align:="" start;="" text-indent:="" 0px;="" text-transform:="" none;="" white-space:="" widows:="" word-spacing:="" -webkit-text-stroke-="" background-color:="" rgb(255,="" 255,="" 255);="" text-decoration-style:="" initial;="" text-decoration-color:="" initial;"="">网络上有一款比较创新的检测方法，利用 canvas 输出图像的方式。

function canUseWebP() {
    var elem = document.createElement('canvas');

    if (!!(elem.getContext && elem.getContext('2d'))) {
        // was able or not to get WebP representation
        return elem.toDataURL('image/webp').indexOf('data:image/webp') == 0;
    }

    // very old browser like IE 8, canvas not supported
    return false;
}

css 中嵌入的 背景图

.no-webp .elementWithBackgroundImage {
  background-image: url("image.jpg");
}

.webp .elementWithBackgroundImage{
  background-image: url("image.webp");
}

<pliga' 1,="" 'onum'="" 'kern'="" 1;="" color:="" rgb(58,="" 65,="" 69);="" font-family:="" tahoma,="" "helvetica="" neue",="" helvetica,="" arial,="" "hiragino="" sans="" gb",="" sans-serif;="" font-size:="" 16px;="" font-style:="" normal;="" font-variant-ligatures:="" font-variant-caps:="" font-weight:="" letter-spacing:="" 0.12px;="" orphans:="" 2;="" text-align:="" start;="" text-indent:="" 0px;="" text-transform:="" none;="" white-space:="" widows:="" word-spacing:="" -webkit-text-stroke-="" background-color:="" rgb(255,="" 255,="" 255);="" text-decoration-style:="" initial;="" text-decoration-color:="" initial;"="">如果用到 背景图的话，我们可以通过跟元素的 class 来进行图片格式的选择