向这个元素添加向外弯曲的边框:◝◟___◞◜

从我的头顶开始:在元素上绘制下半边框,并在定位的伪元素上绘制左上角和右上角边框.

/* using 4px border, 1em border radius, 1em padding */
nav ul {
  font: medium sans-serif;
  text-align: center;
  padding: 0;
  list-style-type: none;
  border-top: 4px solid;
}
nav li {
  display: inline-block;
  border-left: 4px solid transparent;
  border-right: 4px solid transparent;
  border-bottom: 4px solid transparent;
  border-radius: 0 0 1em 1em;
  padding: 0 1em 1em;
  position: relative;
  top: 1em;
}
nav li:before,
nav li:after {
  display: none;
  content: "";
  position: absolute;
  top: -1em;
  width: 1em;
  height: 1em;
  margin-top: -4px;
}
nav li:before {
  right: 100%;
  border-top: 4px solid;
  border-right: 4px solid;
  border-top-right-radius: 1em;
}
nav li:after {
  left: 100%;
  border-top: 4px solid;
  border-left: 4px solid;
  border-top-left-radius: 1em;
}
nav li:hover {
  border-color: initial;
}
nav li:hover:before,
nav li:hover:after {
  display: block;
}

/* using 4px border, 1em border radius, 1em padding */
nav ul {
  font: medium sans-serif;
  text-align: center;
  padding: 0;
  list-style-type: none;
  border-top: 4px solid;
}
nav li {
  display: inline-block;
  border-left: 4px solid transparent;
  border-right: 4px solid transparent;
  border-bottom: 4px solid transparent;
  border-radius: 0 0 1em 1em;
  padding: 0 1em 1em;
  position: relative;
  top: 1em;
}
nav li:before,
nav li:after {
  display: none;
  content: "";
  position: absolute;
  top: -1em;
  width: 1em;
  height: 1em;
  margin-top: -4px;
}
nav li:before {
  right: 100%;
  border-top: 4px solid;
  border-right: 4px solid;
  border-top-right-radius: 1em;
}
nav li:after {
  left: 100%;
  border-top: 4px solid;
  border-left: 4px solid;
  border-top-left-radius: 1em;
}
nav li:hover {
  border-color: initial;
}
nav li:hover:before,
nav li:hover:after {
  display: block;
}