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CSS

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JavaScript

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HTML 

              
                <h1>The most simple tablist (I could come up with) that does not use JavaScript</h1>

<p>This tablist is based on flexbox and the :has selector.</p>

<div>
  <label>
    Focus this text field and then press the tab key on your keyboard to jump into the active tab. It´s then possible to change the active tab with the arrow keys of the keyboard.
    <input type="text" style="display:block; border: 1px solid currentColor; font-size: 1em;">
  </label>

  <p>You can as well select any tab by clicking on it with the mouse pointer.</p>
</div>

<!-- Below comes the tablist

- Roles are used for accessibility. The tablist role is used as the CSS selector for the tablist.
- The name of the input radioboxes has to be adjusted when there are multiple tablists on one page. For each new tablist it needs to be the same for all radio inputs inside of the tablist, but different to the other tablists of the page.

-->

<div role="tablist">
  <!-- tab 1 -->
  <label role="tab">
    <input type="radio" name="tab" checked>Tab 1
  </label>
  <div role="tabpanel">Tab 1 content</div>

  <!-- tab 2 -->
  <label role="tab">
    <input type="radio" name="tab">Tab 2
  </label>
  <div role="tabpanel">Tab 2 content</div>

  <!-- tab 3 -->
  <label role="tab">
    <input type="radio" name="tab">Tab 3
  </label>
  <div role="tabpanel">Tab 3 content</div>
</div>
!

CSS 

              
                [role="tablist"] {
  --outline: auto Highlight;
  --tab-label-padding: 0.5em;
  --tab-label-radius: 0.15em;
  --tab-label-min-width: 4em;
  --tab-label-active-background: white;
  --tab-content-x-padding: 0;
  --tab-content-y-padding: 1em;
  --tab-border-color: currentColor;
  --tab-border-width: 1px;

  display: flex;
  align-items: flex-end;
  flex-wrap: wrap;
}

/* hide everything, except the tab labels */
[role="tablist"] > *:not(label[role="tab"]) {
  display: none;
}

/* keep the inputs on the page, but make them invisible to allow for keyboard focus */
[role="tablist"] > label[role="tab"] > input[type="radio"] {
  opacity: 0;
  width: 0;
  height: 0;
  margin: 0;
}

/* when using the tab key on the keyboard to focus into the tablist, indicate the default outline */
[role="tablist"] > label[role="tab"]:has(:focus-visible) {
  outline: var(--outline);
  outline-offset: calc(-1 * var(--tab-label-padding));
}

/* tab label */
[role="tablist"] > label[role="tab"] {
  padding: var(--tab-label-padding) var(--tab-label-padding);
  min-width: var(--tab-label-min-width);
  margin-bottom: calc(-1 * var(--tab-border-width));
  border-left: var(--tab-border-width) solid transparent;
  border-top: var(--tab-border-width) solid transparent;
  border-right: var(--tab-border-width) solid transparent;
  cursor: pointer;
}

/* selected tab label */
[role="tablist"] > label[role="tab"]:has(input[type="radio"]:checked) {
  border-left: var(--tab-border-width) solid var(--tab-border-color);
  border-top: var(--tab-border-width) solid var(--tab-border-color);
  border-right: var(--tab-border-width) solid var(--tab-border-color);
  border-radius: var(--tab-label-radius) var(--tab-label-radius) 0 0;
  background: var(--tab-label-active-background);
}

/* tab content */
[role="tablist"] > label[role="tab"]:has(input:checked) + [role="tabpanel"] {
  border-top: var(--tab-border-width) solid var(--tab-border-color);
  order: 99;
  display: block;
  width: 100%;
  position: relative;
  z-index: -1;
  padding: var(--tab-content-y-padding) var(--tab-content-x-padding);
}
!

JS
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