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CSS

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JavaScript

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HTML 

              
                <header class="wrapper flow">
  <h1>50/50 grid layout</h1>
  <form id="size-form">
    <div class="cluster">
      <label for="size">Minimum size of each side:</label>
      <input type="text" id="size" name="size" value="250px" />
      <button type="submit">Update</button>
    </div>
  </form>

  <ul>
    <li>Card width: <span class="tnum" id="card-width"></span>px</li>
    <li>Side width: <span class="tnum" id="side-width"></span>px</li>
  </ul>
</header>

<main class="wrapper">
  <article class="card fifty-fifty">
    <section class="poster">
      <img src='https://images.unsplash.com/photo-1516370873344-fb7c61054fa9?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=M3wzMjM4NDZ8MHwxfHJhbmRvbXx8fHx8fHx8fDE3MDc5MjA5MTF8&ixlib=rb-4.0.3&q=80&w=600' alt='Two people in bathing suits holding surfboards approach the ocean shoreline. One person holds their hand high in the sky.'>
    </section>
    <section class="content flow">
      <h2>Catch some waves</h2>
      <p>Enim nemo, exercitationem ullam voluptatum vero assumenda qui repellendus dolore.</p>
    </section>
  </article>

  <article class="card fifty-fifty">
    <section class="poster">
      <img src='https://images.unsplash.com/photo-1559339352-11d035aa65de?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=M3wzMjM4NDZ8MHwxfHJhbmRvbXx8fHx8fHx8fDE3MDc5MjU4MTh8&ixlib=rb-4.0.3&q=80&w=600' alt='Beautiful outdoor patio dining area with the ocean and mountain in the background'>
    </section>
    <section class="content flow">
      <h2>Stay for the views</h2>
      <p>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Officia illo voluptatibus.</p>
    </section>
  </article>
</main>
!

CSS 

              
                @import url("https://fonts.googleapis.com/css2?family=Inter:wght@100..900&display=swap");

:root {
  --text-1: #f8fafc;
  --text-2: #cbd5e1;
  --surface-1: #020617;
  --surface-2: #0f172a;
  --surface-3: #1e293b;
  --surface-4: #334155;
  --button-surface: #1d4ed8;
  --gradient: linear-gradient(
    to bottom,
    hsla(0 0% 0% / 0) 0%,
    hsla(0 0% 100% / 0.065) 100%
  );
}

.fifty-fifty {
  display: grid;
  grid-template-columns: repeat(
    auto-fit,
    minmax(var(--min-inline-size, 250px), 1fr)
  );
}

.card {
  overflow: hidden;
  width: 100%;
  border-radius: 0.5rem;
  background-image: var(--gradient);
  box-shadow: inset 0 0 0 1px var(--surface-3);
}

.card p {
  color: var(--text-2);
}

.card img {
  aspect-ratio: 3 / 2;
  width: 100%;
  height: 100%;
  vertical-align: bottom;
  object-fit: cover;
}

.content {
  padding: 2rem;
  background-image: var(--surface-gradient);
}

.flow > * + * {
  margin-top: 1rem;
}

.wrapper {
  width: min(40rem, 100%);
  margin-inline: auto;
}

.cluster {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 0.2rem 0.5rem;
}

label {
  display: block;
}

.tnum {
  --_offset: 0.04em;

  padding-inline: var(--_offset);
  letter-spacing: calc(var(--_offset) * -1);
  font-variant-numeric: tabular-nums;
  font-weight: 600;
}

:where(input, button) {
  font: inherit;
  color: inherit;
  padding: 0.25rem 0.5rem;
  border: 1px solid var(--surface-4);
  background-color: var(--surface-2);
  border-radius: 0.15rem;
}

input[id="size"] {
  inline-size: 5em;
}

button {
  padding-inline: 0.75rem;
  border-color: var(--button-surface);
  background-color: var(--button-surface);
  background-image: var(--gradient);
}

button:active {
  translate: 0 1px;
}

* {
  box-sizing: border-box;
}

ul {
  padding-inline-start: 1em;
}

:where(h2, p) {
  margin: 0;
}

:where(h1, h2) {
  font-weight: 600;
  letter-spacing: -0.015em;
  line-height: normal;
}

h1 {
  font-size: 1.8rem;
}

h2 {
  font-size: 1.5rem;
}

a {
  color: #38bdf8;
}

header {
  margin-block-end: 2rem;
}

main {
  display: grid;
  gap: 1rem;
  grid-auto-rows: 1fr;
}

body {
  margin: 0;
  margin-block-end: 4rem;
  padding: 2rem;
  font-family: "Inter", sans-serif;
  color: var(--text-1);
  line-height: 1.5;
  background-color: var(--surface-1);
}
!

JS 

              
                let root = document.documentElement;
let form = document.getElementById("size-form");
let submitBtn = form.querySelector("button");
let sizeInput = document.getElementById("size");
let cards = document.querySelectorAll(".card");
let card = cards[0];
let side = card.children[0];
let cardWidthValue = document.getElementById("card-width");
let sideWidthValue = document.getElementById("side-width");

function reportSizes() {
  cardWidthValue.textContent = card.clientWidth;
  sideWidthValue.textContent = side.clientWidth;
}

function handleSubmit(e) {
  e.preventDefault();

  let duration = 400;
  let keyframes = {
    backgroundColor: ["var(--surface-3)", "var(--surface-3)"],
    offset: [0.1, 0.4],
    easing: "ease-out"
  };

  root.style.setProperty("--min-inline-size", sizeInput.value);

  sizeInput.animate(keyframes, duration);

  for (let card of cards) {
    card.animate(keyframes, duration);
  }

  reportSizes();
}

window.addEventListener("resize", reportSizes);
form.addEventListener("submit", handleSubmit);

reportSizes();
!
999px

Console