abc.htm

<div id="a"></div>

<script>
  var size = 20;

  // loop through row
  for(var row=0; row < size; row++){

    // loop through column
    for(var column=0; column < size; column++){

      if(
        row == column ||  // Coords 1,1 2,2 etc (first line)
        (size - column) == row // 1, size -1    2,size-2 etc
      ){
        console.log('x');
        document.write("X");
      }else{
        document.write("&nbsp;");
      }

    }
    
    // new line after each row
    document.write( "<br />");
  }
</script>