DOMContentLoaded

浏览器加载页面是按从上到下顺序加载的，如果想要在页面加载完成时被触发脚本，可以监听这两个事件 ： load 和 DOMContentLoaded

load
load事件是页面 "loading complete"的标志

window.onload = function (){
  //do something
}

缺点
在所有资源完全加载后才被触发，包括图片，会导致比较严重的延迟
因此这个方法很少使用，开发人员又制定了DOMContentLoaded事件

DOMContentLoaded
DOMContentLoaded在DOM加载之后及资源加载之前被触发，不支持IE8及其以下

document.addEventListener( "DOMContentLoaded", ready, false)

Hacks for IE<9

function bindReady(handler){
 
    var called = false
    
    // 确保handler只执行一次
    function ready() { 
        if (called) return
        called = true
        handler()
    }

    if ( document.addEventListener ) { 
        // native event
        document.addEventListener( "DOMContentLoaded", ready, false )
    } else if ( document.attachEvent ) {  
        // IE
        try {
            var isFrame = window.frameElement != null
        } catch(e) {}

        //  IE, the document is not inside a frame
        if ( document.documentElement.doScroll && !isFrame ) {
            function tryScroll(){
                if (called) return
                try {
                    document.documentElement.doScroll("left")
                    ready()
                } catch(e) {
                    setTimeout(tryScroll, 10)
                }
            }
            tryScroll()
        }

        // IE, the document is inside a frame
        document.attachEvent("onreadystatechange", function(){
            if ( document.readyState === "complete" ) {
                ready()
                this.onreadystatechange = null
            }
        })
    }

    // Old browsers
    if ( window.addEventListener ) {
        window.addEventListener('load', ready, false)
    } else if ( window.attachEvent ) {
        window.attachEvent('onload', ready)
    } else {
        // very old browser, copy old onload
        var fn = window.onload 
        // replace by new onload and call the old one
        window.onload = function() { 
            fn && fn()
            ready()
        }
    }
}

主流框架中DOMContentLoaded事件的实现：
http://www.html5jscss.com/mian_ready.html