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CSS

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HTML 

              
                <main>   
  <section>
    <h2>1. Borders on <code>::before</code> and <code>::after</code></h2>
    <p>This is the classic approach with three transparent and one colored border. It has the disadvantage that the clickable zone of a link is still a rectangle. Go for instance to the top left corner of the second link. You will notice that you are actually already on the third link.</p>
    <ul class="border-as-triangle">
      <li>
        <a href="#1">Link</a>
      </li>
      <li>
        <a href="#2">Link</a>
      </li>
      <li class="active">
        <span class="a">Active</span>
      </li>
      <li>
        <a href="#4">Link</a>
      </li>
      <li>
        <span class="a">No link</span>
      </li>
    </ul>
  </section>
  
  <section>
    <h2>2. Skewing and rotating <code>::after</code></h2>
    <p>With this approach the clickable zone is identical to the visual representation of a link - you move outside of the triangle and are on the next link.</p>
    <ul class="after-as-rhombus">
      <li>
        <a href="#1">Link</a>
      </li>
      <li>
        <a href="#2">Link</a>
      </li>
      <li class="active">
        <span class="a">Active</span>
      </li>
      <li>
        <a href="#4">Link</a>
      </li>
      <li>
        <span class="a">No link</span>
      </li>
    </ul>
  </section>
  <section>
    <h2>3. Cutting off with <code>clip-path: polygon()</code></h2>
  <p>A third way to achieve this is to use <code>clip-path: polygon()</code>. Browser support meanwhile is <a href="https://caniuse.com/#search=clip-path" target="_blank">OK-ish</a>.</p>
    <ul class="clip-path-polygon">
      <li>
        <a href="#1">Link</a>
      </li>
      <li>
        <a href="#2">Link</a>
      </li>
      <li class="active">
        <span class="a">Active</span>
      </li>
      <li>
        <a href="#4">Link</a>
      </li>
      <li>
        <span class="a">No link</span>
      </li>
    </ul>
  </section>
</main>
!

CSS 

              
                /// colors
$col-1: #336;
$col-2: #babae7;
$col-3: #003;

/// navi elements
ul {
  padding: 0 50px 0 0;
  list-style: none;
  overflow: hidden;
  display: flex;
  font-size: 1.3rem;
}

li {
  display: block;
  white-space: nowrap;
  &:first-of-type {
    a,
    .a {
      padding-left: 35px;
      border-left: 1px solid $col-1;
    }
  }
  &.active {
    a,
    .a {
      background: $col-3;
      color: $col-2;
    }
  }
  .a {
    cursor: default;
  }
}

ul {
  a,
  .a {
    border-top: 1px solid $col-1;
    border-bottom: 1px solid $col-1;
    padding: 0 15px 0 60px;
    display: inline-block;
    position: relative;
    line-height: 80px;
    text-decoration: none;
    color: $col-1;
    background: $col-2;
  }
  a {
    &:hover {
      color: $col-2;
      background: $col-1;
    }
  }
}

/// border as a triangle technique
.border-as-triangle {
  li {
    &.active {
      a,
      .a {
        &::before {
          border-left-color: $col-3;
        }
        &::after {
          border-left-color: $col-2;
        }
        &:hover {
          &::before {
            border-left-color: $col-3;
          }
        }
      }
    }
    a {
      &:hover {
        &::before {
          border-left-color: $col-1;
        }
        &::after {
          border-left-color: $col-2;
        }
      }
    }
    a,
    .a {
      &::before,
      &::after {
        content: "";
        display: block;
        width: 0;
        height: 0;
        border-top: 41px solid transparent;
        border-bottom: 41px solid transparent;
        position: absolute;
        top: 50%;
        margin-top: -41px;
        left: 100%;
      }
      &::before {
        border-left: 30px solid $col-2;
        z-index: 2;
      }
      &::after {
        border-left: 30px solid $col-1;
        margin-left: 1px;
        z-index: 1;
      }
    }
  }
}

// Transform to rhombus approach
.after-as-rhombus {
  li {
    &.active {
      a,
      .a {
        &::after {
          border-color: $col-2;
        }
      }
    }

    /// link stacking order from highest to lowest
    $countLinks: 4;
    $highest: $countLinks;
    @for $i from 1 through $countLinks {
      &:nth-of-type(#{$i}) {
        z-index: $highest;
      }
      $highest: $highest - 1;
    }
  }
  a {
    &:hover {
      &::after {
        border-color: $col-2;
      }
    }
  }
  a,
  .a {
    &::after {
      content: "";
      display: block;
      height: 95px;
      width: 95px;
      transform: rotate(45deg) skew(10deg, 10deg);
      position: absolute;
      top: -7px;
      z-index: -1;
      right: -21px;
      z-index: -1;
      background: inherit;
      border: 1px $col-1 solid;
    }
  }
}

//  Cutting off with clip-path: polygon()
.clip-path-polygon {
  li {
    &.active {
      a,
      .a {
        &::after {
          background: $col-2;
        }
      }
    }

    /// link stacking order from highest to lowest
    $countLinks: 4;
    $highest: $countLinks;
    @for $i from 1 through $countLinks {
      &:nth-of-type(#{$i}) {
        z-index: $highest;
      }
      $highest: $highest - 1;
    }
  }
  a {
    &:hover {
      &::after {
        background: $col-2;
      }
    }
  }
  a,
  .a {
    &::before,
    &::after {
      content: "";
      display: block;
      height: calc(100% + 2px); // + 2 for parent borders
      width: 120px;
      position: absolute;
      top: -1px;
      right: -120px;
      clip-path: polygon(0 0, 0% 100%, 30px 50%);
    }
    &::before {
      right: -119px;
      background: inherit;
      z-index: 1;
    }
    &::after {
      right: -120px;
      background: $col-1;
    }
  }
}

/// some basic setup
html {
  box-sizing: border-box;
}

*,
*:before,
*:after {
  box-sizing: inherit;
}

body {
  padding: 0 2rem;
  font: 14px sans-serif;
}

main {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
}

section {
  width: 48%;
}

p {
  min-height: 35px;
}
@import url("https://fonts.googleapis.com/css?family=Roboto|Roboto+Mono&display=swap");

:root {
  --background-color: hsl(228, 8%, 12%);
  --text-color: hsl(0, 0%, 85%);
  --border-color: hsl(228, 8%, 20%);
  --transparency-bg: url(data:image/gif;base64,R0lGODlhDAAMAHAAACH/C05FVFNDQVBFMi4wAwEAAAAh+QQICgAAACwAAAAADAAMAIDX19f///8CFoQfqYeabNyDMkBQb81Uat85nxguUAEAOw==);
  --border-radius: .3rem;
  --regular-font: Roboto, sans-serif;
  --monospace-font: 'Roboto Mono', monospace;
  --form-field-bg: hsl(0, 0%, 95%);
  --form-field-color: hsl(228, 8%, 12%);
  --link-color: hsl(206, 89%, 45%);
}

html {
  font-size: 62.5%;
  box-sizing: border-box;
}

a {
  color: var(--link-color);
}

*,
*:before,
*:after {
  box-sizing: inherit;
}

body {
  font: 1.4rem/1.5 var(--regular-font);
  background: var(--background-color);
  color: var(--text-color);
  margin: 0;
}

h1 {
  font-size: 2.4rem;
}

input, textarea, pre, select, [data-preview] {
  background: var(--form-field-bg);
  color: var(--form-field-color);
}

code, input, textarea, pre, select, [data-preview] {
  font-family: var(--monospace-font);  
}

input, textarea, pre, select, fieldset, [data-preview] {
  border: .1rem var(--border-color) solid;
}

pre {
  color: hsl(0, 0%, 15%);
  background: hsl(0, 0%, 85%);
  padding: 1rem;
  max-height: calc(100vh - 40px);
  overflow: auto;
}
!

JS
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