js fade tab tab

code show as below

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<style>
    body {font-family: "Lato", sans-serif;}

    /* Style the tab */
    div.tab {
        overflow: hidden;
        border: 1px solid #ccc;
        background-color: #f1f1f1;
    }

    /* Style the buttons inside the tab */
    div.tab button {
        background-color: inherit;
        float: left;
        border: none;
        outline: none;
        cursor: pointer;
        padding: 14px 16px;
        transition: 0.3s;
        font-size: 17px;
    }

    /* Change background color of buttons on hover */
    div.tab button:hover {
        background-color: #ddd;
    }

    /* Create an active/current tablink class */
    div.tab button.active {
        background-color: #ccc;
    }

    /* Style the tab content */
    /*.tabcontent {*/
        /*display: none;*/
        /*padding: 6px 12px;*/
        /*border: 1px solid #ccc;*/
        /*border-top: none;*/
    /*}*/

    /* Style the tab content */
    .tabcontent {
        display: none;
        padding: 6px 12px;
        -webkit-animation: fadeEffect 1s;
        animation: fadeEffect 1s;
    }

    /* Fade in tabs */
    @-webkit-keyframes fadeEffect {
        from {opacity: 0;}
        to {opacity: 1;}
    }

    @keyframes fadeEffect {
        from {opacity: 0;}
        to {opacity: 1;}
    }
</ Style > 
< body > 
< the p- > Click on each tab to view content: </ the p- >

<div class="tab">
    <button id="defaultOpen" class="tablinks" onclick="openCity(event, 'London')">London</button>
    <button class="tablinks" onclick="openCity(event, 'Paris')">Paris</button>
    <button class="tablinks" onclick="openCity(event, 'Tokyo')">Tokyo</button>
</div>

<div id="London" class="tabcontent">
    <h3>London</h3>
    <p>London is the capital city of England.</p>
</div>

<div id="Paris" class="tabcontent">
    <h3>Paris</h3>
    <p>Paris is the capital of France.</p>
</div>

<div id="Tokyo" class="tabcontent">
    <h3>Tokyo</h3>
    <p>Tokyo is the capital of Japan.</p>
</div>
</body>
</html>
<script>
    function openCity(evt, cityName) {
        var i, tabcontent, tablinks;

        // remove block the effect of all tab-content, tab-links first remove active. 
        tabContent = document.getElementsByClassName ( " tabContent " );
         for (I =  0 ; I < tabcontent.length; I ++ ) {
            tabcontent[i].style.display = "none";
        }
        tablinks = document.getElementsByClassName("tablinks");
        for (i = 0; i < tablinks.length; i++) {
            tablinks[i].className = tablinks[i].className.replace(" active", "");
        }

        // character string corresponding to the tab-content and tab-links plus effect. 
        document.getElementById (cityName) .style.display =  " Block " ;
        evt.currentTarget.className += " active";

//        tabcontent[i].style.display = "none";
    }

    // trigger id = "defaultOpen" click event of a default so that I tabcontent display. 
    document.getElementById ( " defaultOpen " ) .click ();
     
//     var tabContent = document.getElementsByClassName ( "tabContent"); 
//     tabContent [0] = .style.display "Block";
//
//    var tablinks = document.getElementsByClassName("tablinks");
//    tablinks[0].className += " active";
</script>

II. Pop fade effect

If you want to fade in the pop-up animation. Put above tabcontent plus animation style property. And add animation effects fadeEffect definition.

/* Style the tab content */
.tabcontent {
    display: none;
    padding: 6px 12px;
    -webkit-animation: fadeEffect 1s;
    animation: fadeEffect 1s;
}

/* Fade in tabs */
@-webkit-keyframes fadeEffect {
    from {opacity: 0;}
    to {opacity: 1;}
}

@keyframes fadeEffect {
    from {opacity: 0;}
    to {opacity: 1;}
}

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