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;
                    }

Fixed at corner

See also

Cookie banner

Corner ribbon

Fixed at side