html从内表中删除边框

i would like to remove the border from the sides and the bottom of my inner table and stretch it so that it becomes as long as the outer table. I have tried using border-right: none; in my css for bottom, right, and left but the border on those sides of the inner table always stay no matter how wide I make the inner table to meet the outer table. What html/css can I use to hide the border of my inner table but keep the border around the tds?i would like to remove the border from the side