<header>
  <nav>
    <button id="nav-toggle" type="button">
      <span>Navigation</span>
    </button>
    <div id="navigation-content">
      <ul>
        <li><a class="menulink" href="#">Placeholder Link 1</a></li>
        <li><a class="menulink" href="#">Placeholder Link 2</a></li>
        <li><a class="menulink" href="#">Placeholder Link 3</a></li>
      </ul>
    </div>
  </nav>
  <h1>Title</h1>
  <p>Some placeholder text</p>
</header>
<main>
  <section>
    <h2>Section title 1</h2>
    <p>Some content</p>
  </section>
  <section>
    <h2>Section title 2</h2>
    <p>Some content</p>
  </section>
  <section>
    <h2>Section title 3</h2>
    <p>Some content</p>
  </section>
</main>

:root {
  --dark-blue-color: rgb(0 31 51);
  --white-color: rgb(255 215 208);
  --menu-border: 3px solid var(--dark-blue-color);
}

*,
*::before,
*::after {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  width: 100%;
  overflow-x: hidden;
  font-size: 1.2em;
  background-color: tomato;
  color: rgb(32 5 0);
}

ul {
  list-style: none;
}

header {
  height: 90vh;
  display: grid;
  background-color: rgb(0 138 169);
}

nav {
  margin: 20px;
  margin-left: auto;
  display: grid;
  grid-template-columns: 1fr;
  grid-template-rows: repeat(2, 1fr);
  justify-items: stretch;
  height: 200px;
}

#nav-toggle {
  width: 50px;
  margin-left: auto;
  grid-row: 1 / span 1;
  font-size: 1.5em;
  height: 50px;
  cursor: pointer;
  background-color: var(--white-color);
  color: var(--white-color);
  position: relative;
}

#nav-toggle span {
  display: inline-block;
  position: absolute;
  z-index: -1;
}

#nav-toggle::before,
#nav-toggle::after {
  content: "";
  position: absolute;
  width: 26px;
  height: 26px;
  top: 10px;
  left: 9px;
  font-size: 1.5em;
}

#nav-toggle::before {
  z-index: -1;
  background: linear-gradient(
      45deg,
      rgb(0 0 0 / 0) 0 42%,
      var(--dark-blue-color) 0 58%,
      rgb(0 0 0 / 0) 0 100%
    ),
    linear-gradient(
      135deg,
      var(--white-color) 0 42%,
      var(--dark-blue-color) 0 58%,
      var(--white-color) 0 100%
    );
}

#nav-toggle::after {
  background: conic-gradient(
      var(--white-color) 0.25turn,
      var(--dark-blue-color) 0.25turn 0.5turn,
      var(--dark-blue-color) 0.5turn 0.75turn,
      var(--dark-blue-color) 0.75turn
    ) -50px -14px / 96px 10px repeat;
}

#navigation-content {
  z-index: -1;
  position: relative;
  background-color: var(--white-color);
  height: 50px;
}

#navigation-content::before,
#navigation-content::after {
  content: "";
  position: absolute;
  top: -51px;
}

#navigation-content::before {
  width: calc(100% - 51px);
  height: 50px;
  z-index: 0;
}

#navigation-content::after {
  width: 100%;
  height: calc(100% + 51px);
  z-index: -1;
  cursor: pointer;
}

li {
  width: 100%;
  border-top: var(--menu-border);
}

#nav-toggle:hover,
#nav-toggle:focus {
  outline: var(--menu-border);
  color: var(--dark-blue-color);
}

#nav-toggle:focus::before,
nav:focus-within #nav-toggle::before {
  z-index: 5;
}

#nav-toggle:focus ~ #navigation-content,
#navigation-content:focus-within {
  height: auto;
  border: var(--menu-border);
  border-top: 0;
  box-shadow: -5px 5px 10px var(--dark-blue-color);
  right: -3px;
  z-index: 10;
}

.menulink {
  display: inline-block;
  width: 100%;
  padding: 0 20px;
  color: rgb(32 5 0);
  line-height: 50px;
  z-index: -1;
  text-decoration: none;
}

.menulink:hover,
.menulink:focus {
  background-color: var(--dark-blue-color);
  color: var(--white-color);
  text-decoration: underline;
}

header h1,
header p {
  place-self: center;
}

section {
  height: 100vh;
}

h1,
h2,
p {
  padding: 30px;
  text-align: center;
}

/*
 * https://css-tricks.com/click-outside-detector/#comment-1770691
 */