CODE EXAMPLE FOR CSS

fixed table header css

Add table id="mainTable" then take this css   

#mainTable>tbody  {
        display: block;
        height: 400px;
        overflow: auto;
    }

    #mainTable thead, tbody tr {
        display: table;
        width: 100%;
        table-layout: fixed;
    }

    #mainTable thead {
        width: calc( 100% - 1em );
    }