<!DOCTYPE html>
<html>
<head>
<style>
    .container {
        position: relative;
        width: 30px;
        height: 30px;
        margin: 20px;
        border: 1px solid silver;
    }

    .box {
        position: absolute;
        background-color: gray;
    }
    </style>
</head>
<body>
    <div class="container">
        <div class="box" style="left: 1px; top: 1px; width: 10px; height: 10px; box-shadow: 0 0 0 1px blue;"></div>
    </div>

    <div class="container">
        <div class="box" style="left: 1px; top: 1px; width: 10px; height: 10px; box-shadow: 0 0 0 0.5px blue"></div>
    </div>

    <div class="container">
        <div class="box" style="left: 1px; top: 1px; width: 10.5px; height: 10.5px; box-shadow: 0 0 0 0.5px blue"></div>
    </div>

</body>
</html>