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div, textarea {
    margin: 10px 0;
}
.box-sizing {
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}
.sample {
    width: 200px;
    background: #afa;
}
.dev {
    width: 200px;
    background: #ffa;
    padding: 10px;
    border: 5px solid #aaf;
}
textarea {
    width: 200px;
    padding: 10px;
    border: 5px solid #aaf;
    height: 100px;
}

<div class="sample">sample: width:200px;</div>

<div class="dev box-sizing">div: width:200px; padding:10px; border:5px; box-sizing:border-box;</div>

<textarea class="box-sizing">textarea: width:200px; padding:10px; border:5px; box-sizing:border-box;</textarea>

<div class="dev">div without box-sizing: width:200px; padding:10px; border:5px;</div>

<textarea>textarea without box-sizing: width:200px; padding:10px; border:5px</textarea>