• HTML
• CSS

HTML

<div class="loader"></div>

CSS

*
{
    margin:0;
    padding:0;
    box-sizing:border-box;
}
body
{
    display:flex;
    justify-content:center;
    align-items:center;
    min-height:100vh;
    background:#000;
}
.loader
{
    position:relative;
    width:300px;
    height:10px;
    background:#fff;
    -webkit-box-reflect:below 1px linear-gradient(transparent,#0005);
}
.loader::before
{
    content:'';
    position:absolute;
    inset:0;
    background:linear-gradient(90deg,#fb0094,#0000ff,#00ff00,#ffff00,#fb0094,#0000ff,#00ff00,#ffff00,#fb0094);
    animation:animate 20s linear infinite;
    background-size:500%;
}
.loader::after
{
    content:'';
    position:absolute;
    inset:0;
    background:linear-gradient(90deg,#fb0094,#0000ff,#00ff00,#ffff00,#fb0094,#0000ff,#00ff00,#ffff00,#fb0094);
    animation:animate 20s linear infinite;
    background-size:500%;
    filter:blur(20px);
}
@keyframes animate
{
    0%
    {
        background-position:0 0;
    }
    0%
    {
        background-position:500% 0;
    }
}

/* All the cerdits : https://www.youtube.com/watch?v=rdtTCVzTwSQ */