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/**
 * The problem with this one occurs
 * when the browser window is smaller than 960px.
 * The browser then adds a horizontal scrollbar to the page.
 */
.width {
    width: 960px;
    margin-left: auto;
    margin-right: auto;
    border: 3px solid #73AD21;
}

/**
 * Using max-width instead, in this situation,
 * will improve the browser's handling of small windows.
 * This is important when making a site usable on small devices.
 */
.max-width {
    max-width: 960px;
    margin-left: auto;
    margin-right: auto;
    border: 3px solid #73AD21;
}

/**
 * Credits for the tip: W3Schools
 * https://www.w3schools.com/css/css_max-width.asp
 */

CSS TRICKS

https://css-tricks.com/best-way-implement-wrapper-css/

<div class="width">This div element has width: 960px;</div>
<br />

<div class="max-width">This div element has max-width: 960px;</div>
<br />

<strong>Note:</strong> Drag the browser window to smaller than 960px wide, to see the difference between the two divs!