Input 02

<input class="input-basic" type="text" placeholder="テキスト" />

/* reset */
:where(input).input-basic {
  box-sizing: border-box;
  font: inherit;
  appearance: none;
}

/* snippet */
.input-basic {
  --input-border: #e7e7e7;
  --input-text-color: #364153;
  --input-placeholder-color: #99a1af;
  --input-background: #fff;
  --input-focus-color: #2b36f5;

  width: 100%;
  max-width: 320px;
  padding: 14px 16px;
  font-size: 14px;
  line-height: 1.6;
  color: var(--input-text-color);
  background-color: var(--input-background);
  border: 1px solid var(--input-border);
  border-radius: 8px;

  &::placeholder {
    color: var(--input-placeholder-color);
  }

  &:focus:not(:focus-visible) {
    outline: none;
  }

  &:focus-visible {
    outline: 2px solid var(--input-focus-color);
    outline-offset: 2px;
  }
}

<div class="input-error">
  <input
    class="input-error__field js-input-error"
    id="input-error-field"
    type="text"
    placeholder="テキスト"
    aria-label="テキスト"
    aria-invalid="true"
    aria-describedby="input-error-message"
  />
  <p class="input-error__message" id="input-error-message" role="alert">
    テキストを入力してください
  </p>
</div>

/* reset */
:where(input).input-error__field {
  box-sizing: border-box;
  font: inherit;
  appearance: none;
}

/* snippet */
.input-error {
  --input-border: #e7e7e7;
  --input-text-color: #364153;
  --input-placeholder-color: #99a1af;
  --input-background: #fff;
  --input-error-color: #c93030;
  --input-error-background: #fef2f2;
  --input-focus-color: #2b36f5;

  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 320px;
}

.input-error__field {
  width: 100%;
  padding: 14px 16px;
  font-size: 14px;
  line-height: 1.6;
  color: var(--input-text-color);
  background-color: var(--input-background);
  border: 1px solid var(--input-border);
  border-radius: 8px;
  transition:
    background-color 0.15s ease,
    border-color 0.15s ease;

  &::placeholder {
    color: var(--input-placeholder-color);
  }

  &[aria-invalid="true"] {
    background-color: var(--input-error-background);
    border-color: var(--input-error-color);
  }

  &:focus:not(:focus-visible) {
    outline: none;
  }

  &:focus-visible {
    outline: 2px solid var(--input-focus-color);
    outline-offset: 2px;
  }
}

.input-error__message {
  margin: 0;
  font-size: 12px;
  line-height: 1.6;
  color: var(--input-error-color);
}

(function () {
  function clearErrorState(input, messageEl) {
    if (!input.value.trim()) return;

    input.removeAttribute("aria-invalid");
    input.removeAttribute("aria-describedby");
    messageEl?.setAttribute("hidden", "");
  }

  function initInputError(input) {
    if (input.dataset.inputErrorInit === "true") return;

    input.dataset.inputErrorInit = "true";

    const ariaDescribedby = input.getAttribute("aria-describedby");
    const messageEl = ariaDescribedby
      ? document.getElementById(ariaDescribedby)
      : input.closest(".input-error")?.querySelector(".input-error__message");

    input.addEventListener("input", () => {
      clearErrorState(input, messageEl);
    });
  }

  function initAll() {
    document.querySelectorAll(".js-input-error").forEach(initInputError);
  }

  if (document.readyState === "loading") {
    document.addEventListener("DOMContentLoaded", initAll);
  } else {
    initAll();
  }
})();