div border-radius画圆

不要指定border-width属性：

<html>
<style type="text/css">
div{
    border-style: solid;
    background-color: #f00;
    border-width: 50px;
    border-radius: 50px;
    width: 100px;
    height: 100px;
}

</style>
<body>
    <div></div>
</body>
</html>

效果：

去掉border-width属性

<html>
<style type="text/css">
div{
    border-style: solid;
    background-color: #f00;
    border-radius: 50px;
    width: 100px;
    height: 100px;
    background-color: #000;

}

</style>
<body>
    <div></div>
</body>
</html>

效果：

怎么是106和106？

去掉

border-style: solid;

<html>
<style type="text/css">
div{
    background-color: #f00;
    border-radius: 50px;
    width: 100px;
    height: 100px;
}

</style>
<body>
    <div></div>
</body>
</html>

所以如果你想要在:before中画一个圆，则必须让:before是一个block

<html>
<style type="text/css">
div{
    margin: 400px;
}
div:before{
    content: "";
    display: block;
    background-color: #f00;
    border-radius: 50px;
    width: 100px;
    height: 100px;
}

</style>
<body>
    <div></div>
</body>
</html>