CSS implementation of reading progress bar

    <style>
        html,body{margin:0;}
        header{
            position: fixed;
            top:0;
            height: 125px;
            width: 100%;
            background: white;
        }

        main{
            margin-top: 128px;
        }

        @supports (height: 100vh) { 
            body{   
                background: linear-gradient(to right top, #0089f2 50%, #DDD 50%);
                background-size: 100% calc(100% - 100vh + 129px);
                background-repeat: no-repeat;
            }
            body:before{
                content:&#39;&#39;;
                position: fixed;
                top: 128px;
                bottom: 0;
                width: 100%;
                z-index: -1;
                background: white;
            }
        }
     </style>