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CSS

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JavaScript

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HTML 

              
                    <link href="https://fonts.googleapis.com/css2?family=Quicksand:[email protected];400;500;600;700&display=swap" rel="stylesheet">

<div class="logo">
        <a href="https://codeorum.com/" class="codeorum"><img alt="codeorum" src="https://codeorum.com/images/codeorum-logo-dark.svg"></a>
        <a href="https://zovko.pro/" class="mirko-zovko"><img alt="mirko-zovko" src="https://codeorum.com/images/zovko-logo-dark.svg"></a>
    </div>
    <div class="container">
        <h1>Ultra simple slider/slideshow with pure js and animation effect </h1>
        

        <!-- Nav menu container -->
        <nav class="menu" id="myMenu">
            <ul>
                <!-- active link and hover indicator, menu slider -->
                <div class="m-active"></div>

                <!-- menu links -->
                <li><a class="active" href="#">Home</a></li> <!-- active menu  -->
                <li><a href="#">Gallery</a></li>
                <li><a href="#">About</a></li>         
                <li><a href="#">Contact</a></li>      
            </ul>
        </nav>


    </div>
!

CSS 

              
                body{
    margin: 0;
    padding: 0;
    background: #FFC371;
    background: linear-gradient(to right, #9cecfb, #65c7f7, #0052d4);
    font-family: 'Quicksand', sans-serif;
    color: #262626;
}
*{
    box-sizing: border-box;
}
.logo {
    position: fixed;
    display: -webkit-box;
    display: -ms-flexbox;
    display: flex;
    left: 10px;
    top: 10px;
    flex-direction: row;
    z-index: 1000;
    
    a {
        display: block;
        height: 30px;
        width: auto;
        cursor: pointer;
        margin-right: 10px;
        img {
            height: 100%;
            width: auto;
        }
    }
  }

.container{
    display: flex;
    flex-direction: column;
    width: 100%;
    max-width: 1000px;
    margin: 0 auto;
    padding: 50px 10px 100px 10px;
    h1{
        text-align: center;
        margin-bottom: 30px;
        font-weight: 500;
    }
    h2{
        font-weight: 500;
    }
}

// Nav menu container
.menu{
    display: block;
    width: 100%;
    height: auto;
    margin-top: 50px;
    position: relative;
    // unsorted list style with material shadow
    ul{
        position: relative;
        display: flex;
        margin: 0;
        padding: 0 30px;
        list-style: none;
        width: 100%;
        flex-direction: row;
        align-items: center;
        background-color: #262626;
        border-radius: 6px;
        box-shadow: 0 20px 25px -5px rgba(0,0,0,.15),0 10px 10px -5px rgba(0,0,0,.1);
        @media screen and (max-width: 768px) {
            padding: 0 15px;
        }
        // list element with z-index so that text of the menu can be infront of the menu effect slider
        li{
            z-index: 10;
            // link style
            a{
                display: block;
                text-decoration: none;
                padding: 15px 20px;
                color: #eee;
                font-weight: 500;
                font-size: 18px; 
                @media screen and (max-width: 768px) {
                    font-size: 14px; 
                    padding: 10px 15px;
                }
            }
        }
        // active link and hover indicator, menu slider 
        .m-active{
            position: absolute;
            top: -10px;
            bottom: -10px;
            width: 0px;
            background-color: #097ce7;
            transition: all 0.2s linear;
            // top m-active element part
            &:after{
                content: '';
                display: block;
                position: absolute;
                background-color: #085cac;
                width: 100%;
                height: 10px;
                transform: skew(-45deg, 0);
                left: -5px;
                z-index: -1;
            }
            // bottom m-active element part
            &:before{
                content: '';
                display: block;
                position: absolute;
                background-color: #085cac;
                width: 100%;
                height: 10px;
                transform: skew(45deg, 0);
                left: -5px;
                z-index: -1;
                bottom: 0;
            }
        }
    }
}
!

JS 

              
                
function initMenu(elem){

    // function that returns m-active element on active link
    returnToActiveElement();
    function returnToActiveElement(){
        // find active link and get its width and left offset
        var active_menu = document.querySelectorAll(elem + " .active");
        Array.prototype.forEach.call(active_menu, function (e) {
            var width = e.offsetWidth;
            var left = e.offsetLeft;
             // find m-active element and give him width and left offset of active link
            var active_menu_slider = document.querySelectorAll(elem + " .m-active");
            Array.prototype.forEach.call(active_menu_slider, function (el) {
                el.style.width = width+"px";
                el.style.left =  left+"px";
            });
        });    
    }

    // get all links of the menu
    var menu_list = document.querySelectorAll(elem + " a");
    Array.prototype.forEach.call(menu_list, function (e) {
        // mouseenter function, getting width and left offset of the hover link
        e.addEventListener("mouseenter", function( event ) {   
            var width = e.offsetWidth;
            var left = e.offsetLeft;

            // find m-active element and give him width and left offset of hover link
            var active_menu_slider = document.querySelectorAll(elem + " .m-active");
            Array.prototype.forEach.call(active_menu_slider, function (el) {
                el.style.width = width+"px";
                el.style.left =  left+"px";
            });
          }, false);

          // on mouseleave return m-active element to the active link
          e.addEventListener("mouseleave", function( event ) {  
            returnToActiveElement();        
          }, false);

          // on link click, make that link active
          e.addEventListener("click", function( event ) {  
            if (!event.target.matches(elem+' a')) return;
            else{
                if(!event.target.classList.contains('active')){
                    var active_menu = document.querySelectorAll(elem + " .active");
                    Array.prototype.forEach.call(active_menu, function (el) {
                        el.classList.remove("active");
                    }); 
                    event.target.classList.add("active");
                }
            }    
          });
    });


    // on window resize call returnToActiveElement funciton to change the style 
    window.addEventListener("resize", returnToActiveElement);

}

// start menu
initMenu("#myMenu");
!
999px

Console