[voraa]

document.body.onload = function() {
    const tbl = document.getElementById('iItemTbl'); 
    const tblwidth = tbl.offsetWidth;
    const cellwidth = [];
    const cells = tbl.rows[0].cells;
    for (let i = 0; i < cells.length; i++) {
		cellwidth.push(cells[i].offsetWidth)
    }
    document.getElementById("zzz").innerText = tblwidth +' ' + cellwidth.join(',');
}

или короче

document.body.onload = function() {
    const tbl = document.getElementById('iItemTbl'); 
    const tblwidth = tbl.offsetWidth;
    const cells = tbl.tBodies[0].rows[0].cells;
    const cellwidth = Array.from(cells, (cell) => cells[i].offsetWidth);
    document.getElementById("zzz").innerText = tblwidth +' ' + cellwidth.join(',');
}