<div class="main-container">
<button class="btn">
<p>Button</p>
</button>
<div class="someelem"></div>
</div>
.main-container {
display: flex;
height: 100%;
.btn {
background-color: lightcoral;
border: 0;
p {
transform: rotate(-90deg);
}
}
.someelem {
width: 100%;
background-color:cadetblue;
}
}