<div class="container">
    <div class="content">
        By setting <code>box-sizing</code> to <code>border-box</code>, the <code>width</code> of this box will always be 200 pixels on any browser.
    </div>
</div>
html {
    box-sizing: border-box;
}
*, *:before, *:after {
    box-sizing: inherit;
}

.container {
    & {
        width: 200px;
        padding: 20px;
        background: #eee;
        border: 1px solid black;
    }
    > .content {
        background: #ddd;
    }
}
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