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CSS

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HTML 

              
                <!-- 
Demo for blogpost: A Guide To Accessible Form Validation
https://www.smashingmagazine.com/2023/02/guide-accessible-form-validation/
-->

<div class="g-card">
  <h1 class="g-cardTitle">Field validated on submit</h1>
  <div class="demo-side">
    <form class="jsForm" novalidate>
      <div class="field">
        <label for="email" class="field-label">Email</label>
        <input id="email" type="email" class="field-input jsFieldEmail" aria-describedby="emailError" autocomplete="off" />
        <span class="field-hint">
          <span class="field-error" id="emailError" aria-live="assertive">
            <!-- js -->
          </span>

        </span>
      </div>

      <div class="field">
        <label for="phone" class="field-label">Phone number</label>
        <input id="phone" type="phone" class="field-input jsFieldPhone" aria-required="true" aria-describedby="phoneError phoneHint" autocomplete="off" />

        <span class="field-hint">
          <span class="field-error" id="phoneError" aria-live="assertive">
            <!-- js -->
          </span>
          <span id="phoneHint"> For demo purposes, must start with 9.</span>
        </span>
      </div>

      <div class="form-feedback">
        <p class="form-feedback-sr sr-only" aria-live="assertive">
          <!-- js -->
        </p>
        <p class="form-feedback-msg" aria-hidden="true"></p>
      </div>

      <button type="submit" class="btnMain">Save</button>
    </form>

  </div>
</div>

<!-- -->
<!-- -->
<!-- -->

<footer class="g-footer" hidden>
  <p>Made without coffee by <a href="https://twitter.com/a_sandrina_p" class="u-link">Sandrina Pereira</a>.</p>
</footer>
!

CSS 

              
                .field {
  margin-bottom: 16px;

  &.isInvalid {
    color: var(--theme-error);
  }
}

.field-label {
  display: block;
  line-height: 1;
  margin-bottom: 4px;
}

.field-input {
  max-width: 15rem;
  min-height: 3rem;
  border: none;
  border: 1px solid var(--theme-text_1);
  border-radius: 4px;
  font-size: 1.8rem;

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

  &:focus-visible {
    outline: none;
    box-shadow: var(--theme-focus-shadow);
  }
}

.field-hint {
  display: block;
  font-size: 1.4rem;
  margin-top: 4px;
  color: var(--theme-text_1);
}

.field-error {
  color: var(--theme-error);
}

.form-feedback-msg {
  &.isInvalid {
    border-left: 4px solid var(--theme-error);
    margin-bottom: 16px;
    background-color: #fbe9e9;
    border-radius: 0 4px 4px 0;
    padding: 4px 6px;
  }

  &.isSuccess {
    border-left: 4px solid #10b042;
    margin-bottom: 16px;
    background-color: #e8fdee;
    border-radius: 0 4px 4px 0;
    padding: 4px 6px;
  }
}

.btnMain {
  --btnTxt: var(--theme-bg_0);
  position: relative;
  display: inline-block;
  cursor: pointer;
  min-height: 34px;
  padding: 8px 20px;
  background-color: var(--theme-primary);
  border-radius: 4px;
  border: none;
  color: var(--btnTxt);
  text-align: center;
  transition: color 250ms;

  &:focus {
    outline: none;
    box-shadow: var(--theme-focus-shadow);
  }
}

.vo-quote {
  border-left: 4px solid var(--theme-text_1);
  padding-left: 4px;
  background: #eee;
  border-radius: 0 3px 3px 0;
  margin-top: 12px;
}

// ----- Theme styles ----- //

html {
  font-size: 62.5%;
  --theme-width: 450px;
  --theme-text_0: hsl(0deg 0% 20%);
  --theme-text_1: hsl(0deg 0% 43%);
  --theme-bg_0: hsl(27deg 39% 95%);
  --theme-bg_1: hsl(0deg 0% 100%);
  --theme-primary: hsl(266deg 100% 61%);
  --theme-primary_smooth: hsl(266deg 100% 92%);
  --theme-secondary: hsl(27deg 100% 56%);
  --theme-error: #d52424;
  --theme-focus-shadow: var(--theme-bg_0) 0 0 0 2px,
    var(--theme-secondary) 0 0 0 4px;
}

body {
  background-color: var(--theme-bg_0);
  color: var(--theme-text_0);
  font-family: "IBM Plex Sans", sans-serif;
  font-size: 1.6rem;
  font-weight: 300;
  box-sizing: border-box;
  color: #343434;
  line-height: 1.5;
  padding: 45px 16px 90px; /* for fixed footer */
}

* {
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

body * {
  box-sizing: inherit;
}

p {
  margin: 0;
}

// Global components / Utils
.g-hero {
  text-align: center;
  margin-bottom: 32px;
  margin-bottom: 40px;

  h1 {
    font-size: 2.1rem;
    margin: 0;
  }

  p {
    color: var(--theme-text_1);
    font-size: 1.4rem;
  }
}

.g-card {
  position: relative;
  max-width: var(--theme-width);
  padding: 24px 16px;
  margin: 24px auto 60px;
  border: 1px solid var(--theme-primary);
  border-radius: 4px;
  background-color: var(--theme-bg_1);
  box-shadow: 2px 2px var(--theme-primary_smooth);
  min-height: 100px;
}

.g-cardTitle {
  position: absolute;
  top: -30px;
  left: 0;
  // text-transform: uppercase;
  font-size: 1.6rem;
  margin: 0;
  font-weight: 600;
}

.g-footer {
  position: fixed;
  bottom: 0;
  width: 100%;
  padding: 24px 16px;
  font-size: 1.4rem;
  text-align: center;
  background: var(--theme-bg_1);
}

.u-link {
  --linkClr: var(--theme-primary);
  position: relative;
  text-decoration: underline;
  -webkit-text-decoration-color: var(--linkClr);
  text-decoration-color: var(--linkClr);
  color: inherit;
  z-index: 0;
  white-space: nowrap;

  &::before {
    content: "";
    position: absolute;
    bottom: 0.05em;
    left: -0.1em;
    width: calc(100% + 0.2em);
    height: 1.2em;
    background-color: var(--linkClr);
    border-radius: 3px;
    opacity: 0.2;
    transform: scale(1, 0.2);
    transform-origin: 0 95%;
    z-index: -1;
    transition: transform 175ms ease-out;
  }

  &:hover::before,
  &:focus::before {
    transition-duration: 350ms ease-in-out;
    transform: scale(1, 1);
  }

  &:focus {
    outline: none;
  }
}

.u-link:hover {
  outline: none;
}

/* screen reader only */
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  border: 0;
}
!

JS 

              
                const emailRegex = /\S+@\S+\.\S+/; // has @ and .
const phoneRegex = /^9+/; // starts with 9

const elForm = document.getElementsByClassName("jsForm")[0];
const elEmail = document.getElementsByClassName("jsFieldEmail")[0];
const elPhone = document.getElementsByClassName("jsFieldPhone")[0];

const formErrors = {
  email: false,
  phone: false
};

let hasSubmitted = false;

validateField({
  elField: elEmail,
  validateFn: validateFieldEmail
});

validateField({
  elField: elPhone,
  validateFn: validateFieldPhone
});

function validateField({ elField, validateFn }) {
  let touched = false;

  elField.addEventListener("change", (e) => {
    touched = true; // mark it as touched so that on blur it shows the error.
  });

  elField.addEventListener("keyup", (e) => {
    // remove any error on keyup if existent
    validateFn(e.target, { removeOnly: true });

    if (hasSubmitted) {
      updateSubmitSummary();
    }
  });

  elField.addEventListener("blur", (e) => {
    if (!touched) return;
    // show error if touched
    validateFn(e.target, { live: true });
  });
}

function validateFieldEmail(el, opts) {
  const isEmail = emailRegex.test(el.value);
  const isEmpty = el.value === "";
  const isValid = isEmpty || isEmail;

  updateFieldDOM(el, isValid, "Invalid email.", opts);
  formErrors.email = !isValid;
}

function validateFieldPhone(el, opts) {
  const isEmpty = el.value === "";
  updateFieldDOM(el, !isEmpty, "Phone required.", opts);

  if (isEmpty) {
    formErrors.phone = true;
  } else {
    const isPhoneValid = el.value.match(phoneRegex);
    console.log("isPhoneValid", isPhoneValid);
    updateFieldDOM(el, isPhoneValid, "Invalid phone.", opts);
    formErrors.phone = !isPhoneValid;
  }
}

function updateFieldDOM(el, isValid, errorMessage, opts) {
  const removeOnly = opts?.removeOnly;
  const isLive = opts?.live;
  const elField = el.closest(".field");
  const elError = elField.querySelector(".field-error");

  if (isValid) {
    elField.classList.remove("isInvalid");
    elError.innerText = ""; // It's valid
    el.removeAttribute("aria-invalid");
  } else if (!removeOnly) {
    elField.classList.add("isInvalid");
    el.setAttribute("aria-invalid", "true");
    elError.setAttribute("aria-live", isLive ? "assertive" : "off");
    elError.innerText = errorMessage;
  }
}

function updateSubmitSummary({ isSubmit } = {}) {
  const elSummary = elForm.querySelector(".form-feedback");
  const elSummaryMsg = elSummary.querySelector(".form-feedback-msg");

  // Clear form feedback
  elSummaryMsg.classList.remove("isInvalid");
  elSummaryMsg.classList.remove("isSuccess");
  elSummaryMsg.innerText = "";
  const errorsState = Object.values(formErrors);

  if (errorsState.includes(true)) {
    // Show error msg
    const errorCount = Object.values(formErrors).reduce((total, hasError) => {
      return hasError ? total + 1 : total;
    }, 0);
    const errorMsg =
      errorCount === 1
        ? `Failed to save because ${errorCount} field is invalid.`
        : `Failed to save because ${errorCount} fields are invalid.`;
    elSummaryMsg.classList.add("isInvalid");
    elSummaryMsg.innerText = errorMsg;

    elSummary.querySelector(".form-feedback-sr").innerText = isSubmit
      ? // Set SR error message only on submit to avoid being re-announced
        // every time the error summary visually changes.
        errorMsg
      : "";
  } else if (isSubmit) {
    const successMsg = "Saved with success.";
    elSummary.querySelector(".form-feedback-sr").innerText = successMsg;
    elSummaryMsg.innerText = successMsg;
    elSummaryMsg.classList.add("isSuccess");
  }
}

elForm.addEventListener("submit", (e) => {
  e.preventDefault();
  hasSubmitted = true;

  // Validate again
  validateFieldEmail(elEmail);
  validateFieldPhone(elPhone);

  updateSubmitSummary({ isSubmit: true });
});
!
999px

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