button {
  width: 100px;
  height: 100px;
  border: 2px solid black;
  overflow: hidden;
  outline: none;
  float: left;
/*  border: transparent;*/
/*  margin-top: 0px;
  padding: 0px;*/
}

#board {
  border: 2px solid black;
  position: absolute; /* or absolute */
  top: 50%;
  left: 50%;
  margin-top: -150px;
  margin-left: -150px;
  overflow: hidden;
  outline: none;
  /*border: none;*/
}

#row1, #row2, #row3  {
/*  border: solid green;*/
font-size: 0; 
overflow: hidden;
outline: none;
padding: 0;
height: 100px;
}

h1 {
   text-align: center;
}

h2 {
  text-align: center;
  overflow: hidden;
  outline: none;
}