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HTML

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About the <head>

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CSS

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CSS Base

About CSS Base

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HTML 

              
                <h1>Some countries of Asia</h1>

<div class="container with-sidebar">
  <ul id="tablist" role="tablist" class="tablist sidebar">
    <li role="presentation">
      <a class="tab with-sidebar" role="tab" href="#panel1" id="tab1">
        <img loading="lazy" class="flag sidebar" src="https://cdn.countryflags.com/thumbs/mongolia/flag-button-round-250.png" alt="" />
        <span class="description content">Mongolia</span>
      </a>
    </li>
    <li role="presentation">
      <a class="tab with-sidebar" role="tab" href="#panel2" id="tab2">
        <img loading="lazy" class="flag sidebar" src="https://cdn.countryflags.com/thumbs/kyrgyzstan/flag-button-round-250.png" alt="" />
        <span class="description content">Kyrgystan</span>
      </a>
    </li>
    <li role="presentation">
      <a class="tab with-sidebar" role="tab" href="#panel3" id="tab3">
        <img loading="lazy" src="https://cdn.countryflags.com/thumbs/myanmar/flag-button-round-250.png" alt="" class="flag sidebar" />
        <span class="description content">Myanmar</span></a>
    </li>
    <li role="presentation">
      <a class="tab with-sidebar" role="tab" href="#panel4" id="tab4">
        <img loading="lazy" src="https://cdn.countryflags.com/thumbs/uzbekistan/flag-button-round-250.png" alt="" class="flag sidebar" />
        <span class="description content">Uzbekistan</span></a>
    </li>
  </ul>
  <div class="tabpanel-container content">
    <section class="tabpanel" role="tabpanel" tabindex="0" id="panel1" aria-labelledby="tab1">
      <div class="flow">
        <p>
          Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris
          <a href="#panel1">ornare eros egestas</a>
          lectus lacinia, ut luctus turpis egestas. Aliquam erat volutpat.
          Duis sed nisi metus. Ut sodales dapibus efficitur.
        </p>
        <p>
          Aliquam pulvinar vestibulum mauris, id vehicula sapien euismod et.
          Praesent ac elementum nibh. Suspendisse quis justo eu ante rhoncus
          pretium. Integer dictum diam in sem fringilla lobortis.
          Pellentesque gravida dui id felis hendrerit, a tristique lorem
          pretium. Donec vitae velit posuere, pellentesque sem scelerisque,
          luctus tellus.
        </p>
      </div>
      <figure>
        <img loading="lazy" src="https://images.unsplash.com/photo-1589654615616-6756a5653100?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=870&q=80" alt="A mongolian brown grass field surrounded by snow mountains" />
      </figure>
    </section>
    <section class="tabpanel" role="tabpanel" tabindex="0" id="panel2" aria-labelledby="tab2">
      <div class="flow">
        <p>
          Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris
          ornare eros egestas lectus lacinia, ut luctus turpis egestas.
          Aliquam erat volutpat. Duis sed nisi metus. Ut sodales dapibus
          efficitur.
        </p>
        <p>
          Aliquam pulvinar vestibulum mauris, id vehicula sapien euismod et.
          Praesent ac elementum nibh. Suspendisse quis justo eu ante rhoncus
          pretium. Integer dictum diam in sem fringilla lobortis.
          Pellentesque gravida dui id felis hendrerit, a tristique lorem
          pretium. Donec vitae velit posuere, pellentesque sem scelerisque,
          luctus tellus.
        </p>
      </div>
      <figure>
        <img loading="lazy" src="https://images.unsplash.com/photo-1567886793773-d59bfd7f0aa6?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=1032&q=80" alt="A lake in a forest surrounded by mountains" />
      </figure>
    </section>
    <section class="tabpanel" role="tabpanel" tabindex="0" id="panel3" aria-labelledby="tab3">
      <div class="flow">
        <p>
          Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris
          ornare eros egestas lectus lacinia, ut luctus turpis egestas.
          Aliquam erat volutpat. Duis sed nisi metus. Ut sodales dapibus
          efficitur.
        </p>
        <p>
          Aliquam pulvinar vestibulum mauris, id vehicula sapien euismod et.
          Praesent ac elementum nibh. Suspendisse quis justo eu ante rhoncus
          pretium. Integer dictum diam in sem fringilla lobortis.
          Pellentesque gravida dui id felis hendrerit, a tristique lorem
          pretium. Donec vitae velit posuere, pellentesque sem scelerisque,
          luctus tellus.
        </p>
      </div>
      <figure>
        <img loading="lazy" src="https://images.unsplash.com/photo-1584897356466-858d9b6c53d1?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=774&q=80" alt="A temple in Myanmar with a lot of hot air balloons in the background" />
      </figure>
    </section>
    <section class="tabpanel" role="tabpanel" tabindex="0" id="panel4" aria-labelledby="tab4">
      <div class="flow">
        <p>
          Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris
          ornare eros egestas lectus lacinia, ut
          <a href="#panel4">luctus turpis egestas</a>. Aliquam erat
          volutpat. Duis sed nisi metus. Ut sodales dapibus efficitur.
        </p>
        <p>
          Aliquam pulvinar vestibulum mauris, id vehicula sapien euismod et.
          Praesent ac elementum nibh. Suspendisse quis justo eu ante rhoncus
          pretium. Integer
          <a href="#panel4">dictum diam in sem</a> fringilla lobortis.
          Pellentesque gravida dui id felis hendrerit, a tristique lorem
          pretium. Donec vitae velit posuere, pellentesque sem scelerisque,
          luctus tellus.
        </p>
      </div>
      <figure>
        <img loading="lazy" src="https://images.unsplash.com/photo-1557841621-d9f6673405ed?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=870&q=80" alt="A front view of a temple in Uzbekistan" />
      </figure>
    </section>
  </div>
</div>
!

CSS 

              
                :root {
  --bg: hsl(239deg 12% 92%);
  --component-bg: hsl(0deg 0% 85%);
  --accent: royalblue;
}

/* Css reset https://piccalil.li/blog/a-modern-css-reset/ */

*,
*::before,
*::after {
  box-sizing: border-box;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen,
    Ubuntu, Cantarell, "Open Sans", "Helvetica Neue", sans-serif;
}

body,
h1,
p,
figure {
  margin: 0;
}

ul[role="tablist"] {
  list-style: none;
  padding-left: 0;
}

html:focus-within {
  scroll-behavior: smooth;
}

body {
  min-height: 100vh;
  text-rendering: optimizeSpeed;
  line-height: 1.5;
}

a:not([class]) {
  text-decoration-skip-ink: auto;
}

img,
picture {
  max-width: 100%;
  display: block;
}

@media (prefers-reduced-motion: reduce) {
  html:focus-within {
    scroll-behavior: auto;
  }

  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

/* Layout */

.flow > * + * {
  margin-top: var(--flow-space, 1em);
}

.with-sidebar {
  display: flex;
  flex-wrap: wrap;
  max-width: var(--max-width, 100%);
  gap: var(--gap, 0em);
  margin-inline: auto;
}

.sidebar {
  flex-basis: var(--sidebar-width, 9rem);
  flex-grow: 1;
}

.content {
  display: grid;
  place-items: center;
  flex-basis: 0;
  flex-grow: 999;
  min-width: var(--content-width, min(100%, 43.75rem));
}

/* Global styles */

body {
  background-color: var(--bg);
}

h1 {
  text-align: center;
  margin-block: 0.5em;
  color: var(--accent);
}

p {
  max-width: 80ch;
}

a:not([class]):focus {
  outline: 2px solid var(--accent);
  outline-offset: 0.1875rem;
  border-radius: 0.3125rem;
}

[role="tabpanel"]:focus {
  outline: 4px solid var(--accent);
}

/* Component */

.container {
  max-width: 110ch;
  align-items: start;
  background-color: var(--component-bg);
}

.tablist {
  display: flex;
  flex-wrap: wrap;
  margin: 0.2em;
  gap: 1em 0.2em;
  height: 100%;
}

.tablist li {
  flex: 1 1 4.375rem;
}

.tab {
  --sidebar-width: 2rem;
  --content-width: 60%;
  --gap: 0.1em 0.5em;
  padding: 0.5em 0.2em;
  text-decoration-color: transparent;
  border: 2px solid transparent;
  background-color: whitesmoke;
  color: #121212;
  font-size: clamp(0.88rem, calc(0.78rem + 0.49vw), 1.13rem);
}

.tab[aria-selected="true"] {
  background-color: var(--component-bg);
}

.tab:focus-visible {
  background-color: var(--accent);
  color: whitesmoke;
  outline: 0.2em solid transparent;
}

.flag {
  margin-inline: auto;
  aspect-ratio: 1;
  max-width: 2rem;
  height: auto;
  border-radius: 50%;
}

.description {
  font-weight: 500;
}

.tabpanel-container {
  position: relative;
  min-height: 50vh;
  font-size: clamp(1rem, calc(0.93rem + 0.37vw), 1.19rem);
  background-color: hsl(239deg 12% 92%);
  border: 0.2em solid var(--component-bg);
  isolation: isolate;
}

.tabpanel {
  display: flex;
  position: relative;
  flex-wrap: wrap;
  align-items: center;
  gap: 1em;
  padding: 1.5rem;
  background-color: var(--bg);
}

.tabpanel:nth-child(even) {
  flex-direction: row-reverse;
}

.tabpanel > * {
  flex: 1 1 20rem;
}

.tabpanel:not(:target):not(.visible) {
  display: none;
}
!

JS 

              
                const TABLIST = document.querySelector("#tablist");
const TABS = [...TABLIST.querySelectorAll(".tab")];
const TABPANELS = [...document.querySelectorAll(".tabpanel")];

const showActivePanel = (element) => {
  const selectedId = element.id;
  TABPANELS.forEach((e) => {
    e.hidden = "true";
  });
  const activePanel = document.querySelector(
    `[aria-labelledby="${selectedId}"]`
  );
  activePanel.removeAttribute("hidden");
};

const setSelectedTab = (element) => {
  const selectedId = element.id;
  TABPANELS[0].classList.remove("visible");

  TABS.forEach((e) => {
    const id = e.getAttribute("id");
    if (id === selectedId) {
      e.removeAttribute("tabindex", "0");
      e.setAttribute("aria-selected", "true");
    } else {
      e.setAttribute("tabindex", "-1");
      e.setAttribute("aria-selected", "false");
    }
  });
};

const createArrowNavigation = () => {
  const firstTab = TABS[0];
  const lastTab = TABS[TABS.length - 1];

  TABS.forEach((element) => {
    element.addEventListener("keydown", function (e) {
      if ((e.keyCode || e.which) === 38 || (e.keyCode || e.which) === 37) {
        if (element == firstTab) {
          e.preventDefault();
          lastTab.focus();
        } else {
          e.preventDefault();
          const focusableElement = TABS.indexOf(element) - 1;
          TABS[focusableElement].focus();
        }
      } else if (
        (e.keyCode || e.which) === 40 ||
        (e.keyCode || e.which) === 39
      ) {
        if (element == lastTab) {
          e.preventDefault();
          firstTab.focus();
        } else {
          e.preventDefault();
          const focusableElement = TABS.indexOf(element) + 1;
          TABS[focusableElement].focus();
        }
      }
    });
  });
};

const determineTabindex = () => {
  TABPANELS.forEach((element) => {
    const focusableElements = element.querySelectorAll(
      'button:not([disabled]), [href], input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"]):not([disabled]), details:not([disabled]), summary:not(:disabled)'
    ).length;

    focusableElements
      ? element.setAttribute("tabindex", "-1")
      : element.setAttribute("tabindex", "0");
  });
};

const handleClick = () => {
  TABS.forEach((element) => {
    element.addEventListener("click", function () {
      setSelectedTab(element);
      showActivePanel(element);
    });
  });

  TABS.forEach((element) => {
    element.addEventListener("keydown", function (e) {
      if ((e.keyCode || e.which) === 32) {
        setSelectedTab(element);
        showActivePanel(element);
        element.click();
      }
    });
  });
};

const activateFirstPanel = () => {
  TABS[0].setAttribute("tabindex", "0");
  TABS[0].setAttribute("aria-selected", "true");
  TABPANELS[0].classList.add("visible");
};

const checkInitialSelectedTab = () => {
  const targetedTabPanel = document
    .querySelector(".tabpanel:target")
    .getAttribute("aria-labelledby");
  const selectedTab = document.querySelector(`#${targetedTabPanel}`);
  selectedTab.setAttribute("aria-selected", "true");
  selectedTab.removeAttribute("tabindex");
};

const handleInitialState = () => {
  TABS.forEach((e) => {
    e.setAttribute("tabindex", "-1");
    e.setAttribute("aria-selected", "false");
  });

  window.location.href.indexOf("#panel") === -1
    ? activateFirstPanel()
    : checkInitialSelectedTab();

  determineTabindex();
};

handleClick();
createArrowNavigation();

window.onload = handleInitialState;
!
999px

Console