网站新功能引导镂空蒙版的实现实例页面

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代码

CSS代码：
.cover {
    display: none;
    position: absolute;
    width: 0; height: 0;
    left: 0; top: 0; right: 0; bottom: 0;
    border: 0 solid #000;
    opacity: .75;
    filter: alpha(opacity=75);
    z-index: 9;
    /* 过渡效果 */
    transition: all .25s;
    /* 边缘闪动问题fix */
    box-shadow: 0 0 0 100px #000;
    overflow: hidden;
}
.cover::before {
    content: '';
    width: 100%; height:100%;
    border-radius: 50%;
    border: 400px solid #000;
    position: absolute;
    left: -400px; top: -400px;
    box-shadow: inset 0 0 5px 2px rgba(0,0,0,.75);
}
/* IE7, IE8 img */
.cover > img {
    width: 100%; height: 100%;    
}
HTML代码：
<div id="cover" class="cover"></div>
JS代码：
var coverGuide = function(cover, target) {
    var body = document.body, doc = document.documentElement;
    if (cover && target) {
        // target size(width/height)
        var targetWidth = target.clientWidth,
            targetHeight = target.clientHeight;

        // page size
        var pageHeight = doc.scrollHeight,
            pageWidth = doc.scrollWidth;
        
        // offset of target    
        var offsetTop = target.getBoundingClientRect().top + (body.scrollTop || doc.scrollTop),
            offsetLeft = target.getBoundingClientRect().left + (body.scrollLeft || doc.scrollLeft);
        
        // set size and border-width
        cover.style.width = targetWidth + 'px';
        cover.style.height = targetHeight + 'px';    
        cover.style.borderWidth = 
            offsetTop + 'px ' + 
            (pageWidth - targetWidth - offsetLeft) + 'px ' +
            (pageHeight - targetHeight - offsetTop) + 'px ' + 
            offsetLeft + 'px';
        
        cover.style.display = 'block';
            
        // resize
        if (!cover.isResizeBind) {
            if (window.addEventListener) {
                window.addEventListener('resize', function() {
                    coverGuide(cover, target);
                });    
                cover.isResizeBind = true;
            } else if (window.attachEvent) {
                window.attachEvent('onresize', function() {
                    coverGuide(cover, target);
                });
                cover.isResizeBind = true;
                
                // IE7, IE8 box-shadow hack
                cover.innerHTML = '<img src="guide-shadow.png">';
            }
        }
    }
};

var elCover = document.getElementById('cover');
var arrElTarget = [
    document.getElementsByTagName('a')[0], 
    document.getElementById('backTo'), 
    document.getElementById('image')
], index = 0;

coverGuide(elCover, arrElTarget[index]);

elCover.onclick = function() {
    index++;
    if (!arrElTarget[index]) {
        index = 0;    
    }
    coverGuide(elCover, arrElTarget[index]);
};
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