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HTML 

              
                <input type="checkbox" id="nav">
<a id="close" href="#open"><span class="sr-only">Close Menu</span></a>
<a id="open"  href="#close"><span class="sr-only">Open Menu</span></a>
<header>
  <h1><a href="#home">Toggle Menu w/ Focus-Trap</a></h1>
  <label for="nav">
    <span>Menu</span>
    <span>Dismiss</span>
  </label>
  <nav>
    <a href="#item1">Item One</a>
    <a href="#item2">Item Two</a>
    <a href="#item3">Item Three</a>
    <a href="#item4">Item Four</a>
  </nav>
</header>
<main>
  <h2>An Update of Apple's CSS Menu with Focus Trapping and Autofocus</h2>
  <p>This menu is built upon the ideas of Apple's responsive menu <a id="item1" href="https://developer.apple.com/design/human-interface-guidelines/">here</a>, stripped down without extras like CSS animation.</p>
  <p>It has the following change and - I hope - improvement:</p>
  <ol>
    <li>Toggling between the open and close anchor tag will autofocus between them.</li>
    <li>Content will be hidden when menu is open, preventing content being focused.</li>
  </ol>
  <h3>Trap Focus by Removing Content</h3>
  <p>Without JavaScript, the ideal solution to trap focus would have been the ill-fated <code>nav-index</code> CSS property, alas it's gone.</p>
  <p>So the only other solution is to remove everything else so there's nothing <i>to</i> focus beside the menu.</p>
  <p>Doing this is disorienting for the user though.</p>
  <h3>Tab Order Issue</h3>
  <p>Because the anchors needs to auto-focus (set :target to self) and also set <code>display</code> property for contents, it means the anchor tag must live on the same level as the content elements. This makes the HTML structure a bit weird. The anchor tag must live outside of the header, and because it must alter label state it needs to be placed ahead of the header.</p>
  <p>Unfortunately, this means the anchor tags is placed ahead of the header for tab order, creating confusion if you have any other focusable element in the header besides the menu, a hyperlinked logo, for instance.</p>
  <p>You'd think it can be solved with <code>taborder</code>, but because the default tab order is 0 and negative value removes it from tab order, you cannot selectively place an element ahead of everything else - there's nothing between 0 and negative, a gross oversight in my opinion.</p>
  <p>Of course you can do the opposite, but adding tabindex for every other focusable element on the entire page is an unfeasible proposition.</p>
  <p>So for me the solution as it currently stands is not optimal, I will probably revisit it.</p>
  <a href="#">Focus Test 1</a>
  <p></p>
</main>
<aside>
  <h2>Behaviour</h2>
  <p>Trap focus by removing content while mobile menu is open.</p>
  <a href="#">Focus Test 2</a>
</aside>
<footer>
  Footer
</footer>
!

CSS 

              
                /* 100% height */
html {
    height:100%;
    display: flex;
    flex-direction:column;
}
body {
    width:100%;
    flex-grow:1;
    flex-basis:0;
}
html, body {margin:0;padding:0;}

/* color scheme */
:root {
  --clr-header: #ae92f5;
  --clr-body: #f5f2fb;
  --clr-side: #e0daee;
  --clr-footer: #ae92f5;
  --clr-btn: #4a3ca1;
  --clr-btn-hover: #6353c3;
  --clr-btn-txt: #d1c3f5;
  --clr-btn-txt-hover: white;
  --clr-btn-border: #6e5fcb;
  --clr-body-txt: #2f2c35;
}

body > * {padding: 1rem}
h1 {font-size: 1.4rem; margin:0}
h2 {font-size: 1.2rem}
h3 {font-size: 1.1rem;}

/* pull up menu */
body {
  display:grid;
  grid-template-areas: 
    "header header"
    "main   aside"
    "footer footer";
  grid-template-rows:  auto 1fr auto;
  grid-template-columns: 2fr 1fr;
  font-family: sans-serif;
  color: var(--clr-body-txt);
}

header {grid-area:header; background:var(--clr-header)}
main   {grid-area:main;   background:var(--clr-body)}
aside  {grid-area:aside;  background:var(--clr-side)}
footer {grid-area:footer; background:var(--clr-footer)}

header {
  display:flex;
  flex-wrap:wrap;
  align-items:center;
  justify-content:space-between;
  gap:1rem;
}
h1 a {color:var(--clr-body-txt)}
nav {
  display:flex;
  flex-wrap:wrap;
  justify-content:flex-end;
  gap:.5rem .75rem
}
nav a {
  padding:.3rem .6rem;
  background:var(--clr-btn);
  color: var(--clr-btn-txt);
  text-decoration:none;
  border-radius:.4rem;
}
nav a:hover,
nav a:active,
header label:hover span,
header label:active span  {
  background:var(--clr-btn-hover);
  color:var(--clr-btn-txt-hover);
}

#open,
#close,
input#nav,
header label {display:none}

@media (max-width: 768px) {
  body {
    grid-template-areas: 
      "header toggle"
      "nav nav"
      "main main"
      "aside aside"
      "footer footer";
    grid-template-columns: 1fr auto;
    grid-template-rows: auto auto 1fr;
  }
  /* open up header */
  header {display:contents}
  /* use overlapping pseudo element for background */
  header:before {
    content:"";
    grid-column:header/toggle;
    grid-row:header;
    background:var(--clr-header);
  }
  h1 {
    grid-area:header;
    padding:1rem;
    display:flex;
    align-items:center;
    justify-content:space-between;
    gap:1rem;
  }
  /* keep toggle label span the same size */
  #open,
  #close {
    display:initial;
    grid-area:toggle;
    pointer-events: none;
    cursor: default;
    z-index:1
  }
  .sr-only {
    position:absolute;
    clip-path:inset(0px 0px 99.9% 99.9%);
    overflow:hidden;
    height:1px;
    width:1px;
    padding:0;
    border:0
  }
  header label {
    grid-area:toggle;
    display:grid;
    grid-template-areas:"label";
    font-family:sans-serif;
    font-weight:bold;
    color:white;cursor:pointer;
    user-select:none;
    margin:.5rem 1rem;
  }
  header label span {
    grid-area:label;
    text-align:center;
    background: var(--clr-btn);
    padding: .5rem;
    border-radius:.5rem;
    align-self:center;
    color:var(--clr-btn-txt)
  }
  /* vertical menu styling */
  nav {
    grid-area:nav;
    flex-direction:column;
    gap:0;
    display:none;
  }
  nav a {
    padding:.75rem 1rem;
    border-radius:0
  }
  nav a:not(:last-child) {
    border-bottom:1px solid var(--clr-btn-border);
  }
  /* prevent outline from being cut off */
  #open:focus,
  #close:focus,
  nav a:focus {
    outline-offset:-2px;
  }
  /* toggle logic */
  #nav:checked + header nav,
  #close:target ~ nav {display:flex}

  #close:target ~ header nav,
  #nav:checked ~ header nav {
    grid-area:nav / main / aside / footer;
    justify-content:flex-start;
   background:var(--clr-btn);
   display:flex;
   z-index:1;
  }
  #close:target ~ header ~ * ,
  #nav:checked ~ header ~ * {display:none;}
  
  /* label toggle */
  #nav:not(:checked) ~ header label :last-child  {visibility:hidden}
  #nav:checked ~ header label :first-child {visibility:hidden}
  /* anchor toggle */
  #close,
  #close:target + #open {display:none}
  #close:target {display:initial}
  #close:target ~ header label span:first-child {visibility:hidden}
  #close:target ~ header label span:last-child {visibility:visible}
}
!

JS
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