Fixed at corner

<div class="container">
<!-- Top-left corner -->
<div class="container__corner container__corner--tl">
...
</div>
<!-- Top-right corner -->
<div class="container__corner container__corner--tr">
...
</div>
<!-- Bottom-right corner -->
<div class="container__corner container__corner--br">
...
</div>
<!-- Bottom-left corner -->
<div class="container__corner container__corner--bl">
...
</div>
</div>
.container {
position: relative;
}
.container__corner {
position: absolute;
}
.container__corner--tl {
left: 0;
top: 0;
}
.container__corner--tr {
top: 0;
right: 0;
}
.container__corner--br {
bottom: 0;
right: 0;
}
.container__corner--bl {
bottom: 0;
left: 0;
}

See also

Cookie banner

Corner ribbon

Fixed at side