Add outward curving border to elements like this: ◝◟___◞◜

Question

Problem

My navbar looks like this:

HTML and CSS

nav {
  width: 1040px;
}
nav ul {
  display: block;
  list-style-type: none;
  height: 40px;
  border-top: 3px solid #222;
}
nav ul li {
  width: 130px;
  display: inline-block;
  line-height: 40px;
  text-align: center;
  font-size: 16px;
  font-family: Vollkorn;
  font-weight: 400;
}
nav ul li a {
  display: block;
  text-decoration: none;
  color: #333;
}
nav ul li a:hover,
nav ul li a:focus {
  color: #000;
  box-shadow: 0px 0px 2px black;
  border-radius: 10px;
  border: 2px solid #222;
  border-top: 0px;
  transition: 0.2s ease all;
}

<nav>
  <ul>
    <li><a href="#">Home</a>
    </li>
    <li>
      <a href="#">Random</a>
    </li>
    <li>
      <a href="#">Blog</a>
    </li>
    <li>
      <a href="#">About us</a>
    </li>
    <li>
      <a href="#">Contact Us</a>
    </li>
  </ul>
</nav>

Desired Output

In the above picture, the list item blog is focused or hovered. In hover style, it has a border-radius of 5px. The problem is that I want the top left and right corners, when hovered, to curve outwards like this :

I am able to achieve this result using background image, and also using position styles and z-index. I wanted a CSS border-only solution.

P.S.

The desired output may look undesirable (at least to me), but I just wanted to know if this could be done.

Answer 1

From the top of my head: draw bottom half border on the element and draw top-left and top-right borders on positioned pseudo elements.

/* 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;
}

<nav>
  <ul>
    <li>Home</li>
    <li>Random</li>
    <li>Blog</li>
    <li>About us</li>
    <li>Contact Us</li>
  </ul>
</nav>