<div class="popup-container">
  <label class="popup-button" for="login-popup">Login</label>
  <input type="checkbox" id="login-popup">
  <div class="popup">
    <label for="login-popup" class="transparent-label"></label>
    <div class="popup-inner">
      <div class="popup-title">
        <h6>Login</h6>
        <label for="login-popup" class="popup-close-btn">Close</label>
      </div>
      <div class="popup-content">
        <form action="">
          <ul>
            <li>
              <input type="text" placeholder="Username">
            </li>
            <li>
              <input type="password" placeholder="Password">
            </li>
            <li>
              <button type="submit">Log in</button>
            </li>
          </ul>
        </form>
      </div>
    </div>
  </div>
</div>

* {
  padding: 0;
  margin: 0;
  list-style: none;
  text-decoration: none;
}
body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  padding: 25px;
}

.popup-container {
  display: inline-block;
  .popup-button {
    background: #333;
    line-height: 34px;
    color: #fff;
    padding: 0 20px;
    border-radius: 3px;
    display: block;
    cursor: pointer;
    &:hover {
      background: #444;
    }
  }
  .popup {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(#000,.7);
    z-index: 10;
    //display: none;
    opacity: 0;
    visibility: hidden;
    transition: 250ms all;
    .popup-inner {
      width: 400px;
      box-sizing: border-box;
      padding: 20px;
      background: #fff;
      position: absolute;
      left: 50%;
      transform: translate(-50%, -50%);
      top: 150%;
      transition: 250ms all;
      .popup-title {
        display: flex;
        justify-content: space-between;
        align-items: center;
        margin-bottom: 20px;
        h6 {
          font-size: 18px;
          font-weight: 500;
        }
        .popup-close-btn {
          cursor: pointer;
          background: #eee;
          display: block;
          line-height: 30px;
          padding: 0 15px;
          font-size: 14px;
          color: #222;
          border-radius: 3px;
        }
      }
      .popup-content {
        ul {
          li {
            margin-bottom: 10px;
            &:last-child {
              margin-bottom: 0;
            }
            input {
              width: 100%;
              border: 1px solid #ddd;
              border-radius: 3px;
              line-height: 34px;
              padding: 0 15px;
              font-size: 14px;
              box-sizing: border-box;
            }
            button {
              width: 100%;
              line-height: 34px;
              background: #666;
              color: #fff;
              cursor: pointer;
              border-radius: 3px;
              border: none;
              font-size: 14px;
              &:hover {
                background: #444;
              }
            }
          }
        }
      }
    }
    .transparent-label {
      position: fixed;
      top: 0;
      left: 0;
      width: 100%;
      height: 100%;
      z-index: -1;
      cursor: pointer;
    }
  }
  >input {
    display: none;
    &:checked + .popup {
      opacity: 1;
      visibility: visible;
      .popup-inner {
        top: 50%;
      }
    }
  }
}