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<h1>一部だけborder-radius</h1>
<div class="wrap">
<div class="tr com"><div>右上だけ</div></div>
<div class="br com"><div>右下だけ</div></div>
<div class="bl com"><div>左下だけ</div></div>
<div class="tl com"><div>左上だけ</div></div>
</div>

.wrap{margin:0;padding:0;
    display:flex;justify-content: space-around;
    display:-webkit-flex;-webkit-justify-content: space-around;}
h1{font-size:18px;text-align:center}
.com{width:100px;height:100px;background-color:#A72525;
    display: flex;flex-direction: column-reverse;
    display: -webkit-flex;-webkit-flex-direction: column-reverse;}
.com > div{position: absolute;margin-top: 10px;margin-left: 18px;}
.tr{border-top-right-radius:50%}
.br{border-bottom-right-radius:50%}
.bl{border-bottom-left-radius:50%}
.tl{border-top-left-radius:50%}