css polygon

 

 As shown above: to achieve such a corner of the corner cut off.

The idea is: first with a similar background of an area covered with the angle, and then draw a line, then at the notch.

css code:

 .bian{
        height:200px;
        width:500px;
        position:relative;
        border:2px solid #6ca2f6;
        background: rgba(38, 46, 78, 0.6);
    }
    .shape-left-bottom{
            position: absolute;
            bottom: 8px;
            left: -12px;
            z-index: 99;
            width: 0;
            height: 0;
            border-color: transparent transparent transparent #fff;
            border-style: solid;
            border-width: 0 20px 20px;
            transform: rotate(-90deg);
            -ms-transform: rotate(-90deg);
            -webkit-transform: rotate(-90deg);
        }
    .shape-left-bottom:before{
            position: absolute;
            content: '';
            display: inline-block;
            left: -23px;
            bottom: -11px;
            height: 2px;
            width: 27px;
            border-bottom: 2px solid #6ca2f6;
            transform: rotate(-45deg);
            -ms-transform: rotate(-45deg);
            -webkit-transform: rotate(-45deg);
        }

html:

<div class="bian">
    <div class="shape-left-bottom"></div>
</div>