Jonathan Petitcolas - Photo : © Lucille Caballero http://www.lucillecaballero.com

Jonathan Petitcolas

JavaScript and open-source enthusiast

Simulate marquee tag in CSS and Javascript

Several days ago, one of my following asked on Twitter how to simulate marquee tag in full CSS. For the youngest developers who may not know this strongly unadvised tag, it allows to slide a text from right to left. I know it is a deprecated one, because of a huge lack of accessibility. Yet, customers are always right. And, even if you try to convince them that it is a real disaster for readability and accessibility, some simply don’t care. So, here is a CSS alternative.

First, the HTML:

<p class="marquee">Do not use marquee tag for improve accessibility and readability.</p>

And now the related CSS:

@keyframes marquee {
    0%   { text-indent: 430px }
    100% { text-indent: -485px }
}

@-webkit-keyframes marquee {
    0%   { text-indent: 430px }
    100% { text-indent: -485px }
}

.marquee {
    overflow: hidden;
    white-space: nowrap;
    animation: marquee 17s linear infinite;
    -webkit-animation: marquee 17s linear infinite;
}

.marquee:hover {
    -webkit-animation-play-state: paused;
    animation-play-state: paused;
}

This trick deals with a text-indent animation. I do not use a padding animation, to avoid some issues with border-box specifications. Here is a small schema to better explain the value I set:

Marquee limits

As you may see below, this solution is fluider than the old marquee tag:

Moreover, there are two extra features with this solution:

However, previous solution has a major issue: it requires to specify hard-written text indentation extremes. We are going to use some Javascript to change keyframe values on the fly:

function getStringWidth(str) {

    var span = document.createElement("span");
    span.innerText = str;
    span.style.visibility = "hidden";

    var body = document.getElementsByTagName("body")[0];
    body.appendChild(span);
    var textWidth = span.offsetWidth;
    body.removeChild(span);

    return textWidth;
}

function getAnimationRule(animationName) {
    var KEYFRAME_RULE = window.CSSRule.WEBKIT_KEYFRAMES_RULE ||
        window.CSSRule.MOZ_KEYFRAMES_RULE ||
        window.CSSRule.KEYFRAMES_RULE;

    var stylesheets = document.styleSheets;
    for (var i = 0 ; i < stylesheets.length ; i++) {
        var rules = stylesheets[i].cssRules;
        for (var j = 0 ; j < rules.length ; j++) {
            var rule = rules[j];
            if (rule.type == KEYFRAME_RULE && rule.name == "marquee") {
                return rule;
            }
        }
    }
}

function updateMarqueeAmplitude(element) {

    var animationName = "marquee";
    var marqueeRule = getAnimationRule(animationName);
    if (null == marqueeRule) {
        return;
    }

    // remove the old animation (if any)
    element.style.webkitAnimationName = "none";

    var textWidth = getStringWidth(element.innerText);

    // update the values of our keyframe animation
    marqueeRule.deleteRule("0%");
    marqueeRule.deleteRule("100%");
    marqueeRule.insertRule('0% { text-indent: ' + element.offsetWidth + 'px; }');
    marqueeRule.insertRule('100% { text-indent: ' + -textWidth + 'px; }');

    // re-assign the animation (to make it run)
    element.style.webkitAnimationName = animationName;
}

updateMarqueeAmplitude(document.querySelector(".marquee"));

The first function getStringWidth is straightforward: we calculate the length in pixel of a string by hiding it into the DOM. It is a far more reliable way to measure it than to use string length and font size.

The second one, getAnimationRule is far more interesting. This method browses the CSS-OM (the equivalent of DOM for stylesheets) to retrieve animation keyframes declaration.

Finally, updateMarqueeAmplitude. This is the main function of our script. We retrieve the animation, we stop it by changing its name, and then we update the keyframe rules before restarting it.

Here is the related result:

What about performances? As you may have guess, the bottleneck of this application is in the getAnimationRule function. The more stylesheets and declaration you have, the more resource you will consume to retrieve the rule. It is a major issue, especially if you use some big frameworks such as Twitter Bootstrap.

That’s why I do not recommend to use this script to get the extreme values of text-indent. Here are three ways to avoid this bottleneck:

 

comments powered by Disqus