попробуйте использовать display: flex
из change-button
класс
body{
font-size:0px;
}
*{
margin: 0px;
padding: 0px;
box-sizing: border-box;
border:0px solid black;
}
.increase{
height: 100px;
width: 100px;
font-size: 25px;
background-color: rgb(221, 221, 221);
}
.decrease{
height: 100px;
width: 100px;
font-size: 25px;
background-color: rgb(221, 221, 221);
}
.change-button
{
display: flex;
align-items: center;
}
.result{
text-align:center;
height: 100px;
width: 100px;
font-size: 45px;
background-color: black;
color: white;
}
<div class="change-button">
<button class="increase">+</button>
<input type="text" class="result" disabled="false" value="0"/>
<button class="decrease">-</button>
</div>