逻辑

1、首先有一个圆：蓝色的纯净的圆,效果：

 

2、再来两个半圆，左边一个，右边一个将此蓝色的圆盖住，效果：

 

此时将右半圆旋转60°，就会漏出底圆，效果：

代码：

<style>
     /*支持IE9及以上*/
    .circle-bar {margin: 20px; font-size:200px; width: 1em; height: 1em; position: relative;  background-color: #29a6e6; }
    .circle-bar-left,.circle-bar-right { width: 1em; height: 1em;  }
    /*
        这里采用clip剪切了圆，实现左右两个半圆，右半圆在后面
     */
    .circle-bar-right { clip:rect(0,auto,auto,.5em);background-color: #e2e2e2; }
    .circle-bar-left { clip:rect(0,.5em,auto,0); background-color: red;}

    .mask { width: 0.8em; height: 0.8em;  background-color: #fff;text-align: center;line-height: 0.2em; color:rgba(0,0,0,0.5); }
    .mask :first-child { font-size: 0.3em; height: 0.8em; line-height: 0.8em; display: block;  }
    /*所有的后代都水平垂直居中，这样就是同心圆了*/
    .circle-bar * {  position: absolute; top:0; right:0; bottom:0; left:0; margin:auto; }
    /*自身以及子元素都是圆*/
    .circle-bar, .circle-bar > * { border-radius: 50%; }
</style>

<div class="circle-bar">
    <div class="circle-bar-left"></div>
    <div class="circle-bar-right"></div>
     <!--遮罩层，显示百分比-->
    <div class="mask">
        <span class="percent"></span>
    </div>
</div>

<script>
    $(function () {
        var percent = 0;
        var interval = setInterval(function () {
            //            var percent = parseInt($(‘.mask :first-child‘).text());
            percent+=5;
            var baseColor = $(‘.circle-bar‘).css(‘background-color‘);
            if (percent <= 50) {
                $(‘.circle-bar-right‘).css(‘transform‘, ‘rotate(‘ + (percent * 3.6) + ‘deg)‘);
            } else {
                $(‘.circle-bar-right‘).css({
                    ‘transform‘: ‘rotate(0deg)‘,
                    ‘background-color‘: baseColor
                });
                $(‘.circle-bar-left‘).css(‘transform‘, ‘rotate(‘ + ((percent - 50) * 3.6) + ‘deg)‘);
            }
            $(‘.mask :first-child‘).html(percent + "%");
            if (percent == 100) {
                $(‘.circle-bar-right‘).css({
                    ‘transform‘: ‘rotate(0deg)‘,
                    ‘background-color‘: ‘#e2e2e2‘
                });
                $(‘.circle-bar-left‘).css({
                    ‘transform‘: ‘rotate(0deg)‘,
                    ‘background-color‘: ‘#e2e2e2‘
                });
                percent = 0;
                clearInterval(interval);
            }
        }, 1000);
    })
</script>