abc.htm

<script>
  var size = 25;
  var spaces = 6;
  var row=0;

  // Make spaces string
  spaces = new Array(spaces).join("&nbsp;");

  // loop through row
  while(row < size){
    var column=0;
    // loop through column
    while(column < size){

      if(
        row == column ||  // Coords 1,1 2,2 etc (first line)
        (size - column) == row // 1, size -1    2,size-2 etc
      ){
        document.write("X");
      }else{
        document.write(spaces);
      }

      // Increment column counter
      column++;

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

    // Increment row counter
    row++;
  }
</script>