让IE6、7、8兼容@media属性

通常做页面适配的时候，经常会用到@media属性，对不同屏幕范围内的元素设置不同的样式。但是@media属性不兼容IE8及IE8以下的浏览器

解决方法：

直接在页面中引入respond.src.js即可

/*! Respond.js v1.4.2: min/max-width media query polyfill
 * Copyright 2014 Scott Jehl
 * Licensed under MIT
 * http://j.mp/respondjs */

/*! matchMedia() polyfill - Test a CSS media type/query in JS. Authors & copyright (c) 2012: Scott Jehl, Paul Irish, Nicholas Zakas. Dual MIT/BSD license */
/*! NOTE: If you're already including a window.matchMedia polyfill via Modernizr or otherwise, you don't need this part */
(function(w) {
  "use strict";
  w.matchMedia = w.matchMedia || function(doc, undefined) {
    var bool, docElem = doc.documentElement, refNode = docElem.firstElementChild || docElem.firstChild, fakeBody = doc.createElement("body"), div = doc.createElement("div");
    div.id = "mq-test-1";
    div.style.cssText = "position:absolute;top:-100em";
    fakeBody.style.background = "none";
    fakeBody.appendChild(div);
    return function(q) {
      div.innerHTML = '&shy;<style media="' + q + '"> #mq-test-1 {  42px; }</style>';
      docElem.insertBefore(fakeBody, refNode);
      bool = div.offsetWidth === 42;
      docElem.removeChild(fakeBody);
      return {
        matches: bool,
        media: q
      };
    };
  }(w.document);
})(this);

(function(w) {
  "use strict";
  var respond = {};
  w.respond = respond;
  respond.update = function() {};
  var requestQueue = [], xmlHttp = function() {
    var xmlhttpmethod = false;
    try {
      xmlhttpmethod = new w.XMLHttpRequest();
    } catch (e) {
      xmlhttpmethod = new w.ActiveXObject("Microsoft.XMLHTTP");
    }
    return function() {
      return xmlhttpmethod;
    };
  }(), ajax = function(url, callback) {
    var req = xmlHttp();
    if (!req) {
      return;
    }
    req.open("GET", url, true);
    req.onreadystatechange = function() {
      if (req.readyState !== 4 || req.status !== 200 && req.status !== 304) {
        return;
      }
      callback(req.responseText);
    };
    if (req.readyState === 4) {
      return;
    }
    req.send(null);
  }, isUnsupportedMediaQuery = function(query) {
    return query.replace(respond.regex.minmaxwh, "").match(respond.regex.other);
  };
  respond.ajax = ajax;
  respond.queue = requestQueue;
  respond.unsupportedmq = isUnsupportedMediaQuery;
  respond.regex = {
    media: /@media[^{]+{([^{}]*{[^}{]*})+/gi,
    keyframes: /@(?:-(?:o|moz|webkit)-)?keyframes[^{]+{(?:[^{}]*{[^}{]*})+[^}]*}/gi,
    comments: //*[^*]**+([^/][^*]**+)*//gi,
    urls: /(url()['"]?([^/)'"][^:)'"]+)['"]?())/g,
    findStyles: /@media *([^{]+){([Ss]+?)$/,
    only: /(onlys+)?([a-zA-Z]+)s?/,
    minw: /(s*min-widths*:s*(s*[0-9.]+)(px|em)s*)/,
    maxw: /(s*max-widths*:s*(s*[0-9.]+)(px|em)s*)/,
    minmaxwh: /(s*m(in|ax)-(height|width)s*:s*(s*[0-9.]+)(px|em)s*)/gi,
    other: /([^)]*)/g
  };
  respond.mediaQueriesSupported = w.matchMedia && w.matchMedia("only all") !== null && w.matchMedia("only all").matches;
  if (respond.mediaQueriesSupported) {
    return;
  }
  var doc = w.document, docElem = doc.documentElement, mediastyles = [], rules = [], appendedEls = [], parsedSheets = {}, resizeThrottle = 30, head = doc.getElementsByTagName("head")[0] || docElem, base = doc.getElementsByTagName("base")[0], links = head.getElementsByTagName("link"), lastCall, resizeDefer, eminpx, getEmValue = function() {
    var ret, div = doc.createElement("div"), body = doc.body, originalHTMLFontSize = docElem.style.fontSize, originalBodyFontSize = body && body.style.fontSize, fakeUsed = false;
    div.style.cssText = "position:absolute;font-size:1em;1em";
    if (!body) {
      body = fakeUsed = doc.createElement("body");
      body.style.background = "none";
    }
    docElem.style.fontSize = "100%";
    body.style.fontSize = "100%";
    body.appendChild(div);
    if (fakeUsed) {
      docElem.insertBefore(body, docElem.firstChild);
    }
    ret = div.offsetWidth;
    if (fakeUsed) {
      docElem.removeChild(body);
    } else {
      body.removeChild(div);
    }
    docElem.style.fontSize = originalHTMLFontSize;
    if (originalBodyFontSize) {
      body.style.fontSize = originalBodyFontSize;
    }
    ret = eminpx = parseFloat(ret);
    return ret;
  }, applyMedia = function(fromResize) {
    var name = "clientWidth", docElemProp = docElem[name], currWidth = doc.compatMode === "CSS1Compat" && docElemProp || doc.body[name] || docElemProp, styleBlocks = {}, lastLink = links[links.length - 1], now = new Date().getTime();
    if (fromResize && lastCall && now - lastCall < resizeThrottle) {
      w.clearTimeout(resizeDefer);
      resizeDefer = w.setTimeout(applyMedia, resizeThrottle);
      return;
    } else {
      lastCall = now;
    }
    for (var i in mediastyles) {
      if (mediastyles.hasOwnProperty(i)) {
        var thisstyle = mediastyles[i], min = thisstyle.minw, max = thisstyle.maxw, minnull = min === null, maxnull = max === null, em = "em";
        if (!!min) {
          min = parseFloat(min) * (min.indexOf(em) > -1 ? eminpx || getEmValue() : 1);
        }
        if (!!max) {
          max = parseFloat(max) * (max.indexOf(em) > -1 ? eminpx || getEmValue() : 1);
        }
        if (!thisstyle.hasquery || (!minnull || !maxnull) && (minnull || currWidth >= min) && (maxnull || currWidth <= max)) {
          if (!styleBlocks[thisstyle.media]) {
            styleBlocks[thisstyle.media] = [];
          }
          styleBlocks[thisstyle.media].push(rules[thisstyle.rules]);
        }
      }
    }
    for (var j in appendedEls) {
      if (appendedEls.hasOwnProperty(j)) {
        if (appendedEls[j] && appendedEls[j].parentNode === head) {
          head.removeChild(appendedEls[j]);
        }
      }
    }
    appendedEls.length = 0;
    for (var k in styleBlocks) {
      if (styleBlocks.hasOwnProperty(k)) {
        var ss = doc.createElement("style"), css = styleBlocks[k].join("
");
        ss.type = "text/css";
        ss.media = k;
        head.insertBefore(ss, lastLink.nextSibling);
        if (ss.styleSheet) {
          ss.styleSheet.cssText = css;
        } else {
          ss.appendChild(doc.createTextNode(css));
        }
        appendedEls.push(ss);
      }
    }
  }, translate = function(styles, href, media) {
    var qs = styles.replace(respond.regex.comments, "").replace(respond.regex.keyframes, "").match(respond.regex.media), ql = qs && qs.length || 0;
    href = href.substring(0, href.lastIndexOf("/"));
    var repUrls = function(css) {
      return css.replace(respond.regex.urls, "$1" + href + "$2$3");
    }, useMedia = !ql && media;
    if (href.length) {
      href += "/";
    }
    if (useMedia) {
      ql = 1;
    }
    for (var i = 0; i < ql; i++) {
      var fullq, thisq, eachq, eql;
      if (useMedia) {
        fullq = media;
        rules.push(repUrls(styles));
      } else {
        fullq = qs[i].match(respond.regex.findStyles) && RegExp.$1;
        rules.push(RegExp.$2 && repUrls(RegExp.$2));
      }
      eachq = fullq.split(",");
      eql = eachq.length;
      for (var j = 0; j < eql; j++) {
        thisq = eachq[j];
        if (isUnsupportedMediaQuery(thisq)) {
          continue;
        }
        mediastyles.push({
          media: thisq.split("(")[0].match(respond.regex.only) && RegExp.$2 || "all",
          rules: rules.length - 1,
          hasquery: thisq.indexOf("(") > -1,
          minw: thisq.match(respond.regex.minw) && parseFloat(RegExp.$1) + (RegExp.$2 || ""),
          maxw: thisq.match(respond.regex.maxw) && parseFloat(RegExp.$1) + (RegExp.$2 || "")
        });
      }
    }
    applyMedia();
  }, makeRequests = function() {
    if (requestQueue.length) {
      var thisRequest = requestQueue.shift();
      ajax(thisRequest.href, function(styles) {
        translate(styles, thisRequest.href, thisRequest.media);
        parsedSheets[thisRequest.href] = true;
        w.setTimeout(function() {
          makeRequests();
        }, 0);
      });
    }
  }, ripCSS = function() {
    for (var i = 0; i < links.length; i++) {
      var sheet = links[i], href = sheet.href, media = sheet.media, isCSS = sheet.rel && sheet.rel.toLowerCase() === "stylesheet";
      if (!!href && isCSS && !parsedSheets[href]) {
        if (sheet.styleSheet && sheet.styleSheet.rawCssText) {
          translate(sheet.styleSheet.rawCssText, href, media);
          parsedSheets[href] = true;
        } else {
          if (!/^([a-zA-Z:]*//)/.test(href) && !base || href.replace(RegExp.$1, "").split("/")[0] === w.location.host) {
            if (href.substring(0, 2) === "//") {
              href = w.location.protocol + href;
            }
            requestQueue.push({
              href: href,
              media: media
            });
          }
        }
      }
    }
    makeRequests();
  };
  ripCSS();
  respond.update = ripCSS;
  respond.getEmValue = getEmValue;
  function callMedia() {
    applyMedia(true);
  }
  if (w.addEventListener) {
    w.addEventListener("resize", callMedia, false);
  } else if (w.attachEvent) {
    w.attachEvent("onresize", callMedia);
  }
})(this);