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Form Validation States and User Feedback

Validation states are critical UX — users need immediate, clear feedback when their input is correct or incorrect. CSS provides powerful pseudo-classes for native validation, supplemented by JS-applied classes for custom rules.

1. Native CSS Validation Pseudo-Classes​

StatePseudo-ClassWhen Active
Input passes validation:validRequired + not empty; email format OK; number in range
Input fails validation:invalidRequired + empty; bad email; number out of range
Value within min/max:in-rangeNumber type with min/max
Value outside min/max:out-of-range
Field is required:requiredHas required attribute
Field is optional:optionalNo required attribute
Placeholder showing:placeholder-shownField is empty (placeholder visible)
Value matches pattern:user-valid ⚡Valid after user interacted (modern)
Value doesn't match:user-invalid ⚡Invalid after user interacted (modern)

[!NOTE] Use :user-valid and :user-invalid (newer) instead of :valid/:invalid whenever possible — they only apply after the user has interacted with the field, preventing the jarring red border on page load before the user has typed anything.

2. The :user-valid / :user-invalid Pattern (Modern)​

/* Only show validation state after user interaction */
.form-input:user-valid {
  border-color: var(--color-success, #38a169);
  box-shadow: 0 0 0 3px rgba(56, 161, 105, 0.2);
}

.form-input:user-invalid {
  border-color: var(--color-danger, #e53e3e);
  box-shadow: 0 0 0 3px rgba(229, 62, 62, 0.2);
}

/* Fallback for browsers without :user-valid support */
@supports not selector(:user-valid) {
  /* Apply class with JS after first change event */
  .form-input.is-touched:valid   { border-color: var(--color-success); }
  .form-input.is-touched:invalid { border-color: var(--color-danger); }
}

3. Complete Validation State Styles​

/* ── Input states ── */
.form-input:user-valid,
.form-input.is-valid {
  border-color: var(--color-success, #38a169);
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 20 20' fill='%2338a169'%3E%3Cpath d='M16.707 5.293a1 1 0 010 1.414l-8 8a1 1 0 01-1.414 0l-4-4a1 1 0 011.414-1.414L8 12.586l7.293-7.293a1 1 0 011.414 0z'/%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: right 0.875rem center;
  background-size: 1.25rem;
  padding-right: 2.5rem;
}

.form-input:user-invalid,
.form-input.is-invalid {
  border-color: var(--color-danger, #e53e3e);
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 20 20' fill='%23e53e3e'%3E%3Cpath fill-rule='evenodd' d='M18 10a8 8 0 11-16 0 8 8 0 0116 0zm-7 4a1 1 0 11-2 0 1 1 0 012 0zm-1-9a1 1 0 00-1 1v4a1 1 0 102 0V6a1 1 0 00-1-1z' clip-rule='evenodd'/%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: right 0.875rem center;
  background-size: 1.25rem;
  padding-right: 2.5rem;
}

.form-input:user-valid:focus   { box-shadow: 0 0 0 3px rgba(56, 161, 105, 0.25); }
.form-input:user-invalid:focus { box-shadow: 0 0 0 3px rgba(229, 62, 62, 0.25); }

4. Error and Success Messages​

<!-- Invalid state with error message -->
<div class="form-group">
  <label class="form-label" for="email">Email</label>
  <input class="form-input is-invalid" type="email" id="email"
         aria-describedby="email-error" aria-invalid="true">
  <span class="form-error" id="email-error" role="alert">
    Please enter a valid email address.
  </span>
</div>

<!-- Valid state with success message -->
<div class="form-group">
  <label class="form-label" for="username">Username</label>
  <input class="form-input is-valid" type="text" id="username"
         aria-describedby="username-success">
  <span class="form-success" id="username-success">
    ✓ Username is available!
  </span>
</div>
/* Error message */
.form-error {
  display: flex;
  align-items: center;
  gap: 0.3rem;
  font-size: var(--text-sm, 0.875rem);
  color: var(--color-danger, #e53e3e);
  margin-top: 0.25rem;
  animation: shake-error 0.4s ease;
}
.form-error::before { content: '⚠ '; }

/* Success message */
.form-success {
  font-size: var(--text-sm, 0.875rem);
  color: var(--color-success, #38a169);
  margin-top: 0.25rem;
}

/* Shake animation for errors */
@keyframes shake-error {
  0%, 100% { transform: translateX(0); }
  25%       { transform: translateX(-6px); }
  75%       { transform: translateX(6px); }
}

5. Live Password Strength Indicator​

<div class="form-group">
  <label for="password">Password</label>
  <input type="password" id="password" class="form-input">
  <div class="strength-bar">
    <span class="strength-bar__track">
      <span class="strength-bar__fill"></span>
    </span>
    <span class="strength-bar__label">Enter a password</span>
  </div>
</div>
.strength-bar { margin-top: 0.5rem; }

.strength-bar__track {
  display: block;
  height: 4px;
  background: var(--color-border);
  border-radius: 2px;
  overflow: hidden;
}

.strength-bar__fill {
  display: block;
  height: 100%;
  width: 0%;
  border-radius: 2px;
  transition: width 0.4s ease, background 0.4s ease;
}

/* Strength levels — toggled via JS by adding class to .strength-bar */
.strength-bar[data-strength="weak"]   .strength-bar__fill { width: 25%; background: var(--color-danger); }
.strength-bar[data-strength="fair"]   .strength-bar__fill { width: 50%; background: var(--color-warning); }
.strength-bar[data-strength="good"]   .strength-bar__fill { width: 75%; background: hsl(40,90%,55%); }
.strength-bar[data-strength="strong"] .strength-bar__fill { width: 100%; background: var(--color-success); }

.strength-bar__label { font-size: var(--text-sm); color: var(--color-text-muted); margin-top: 0.25rem; display: block; }

6. Required Field Indicator Pattern​

/* Mark required fields automatically */
label:has(+ [required])::after,
label:has(+ [aria-required="true"])::after {
  content: ' *';
  color: var(--color-danger, #e53e3e);
  font-weight: 400;
}

/* Or with a data attribute */
.form-label[data-required]::after {
  content: ' (required)';
  color: var(--color-text-muted);
  font-size: 0.8em;
  font-weight: 400;
}

7. Complete Form with All Validation States​

/* Entire form section feedback when invalid */
.form-section:has(input:invalid:not(:placeholder-shown)) {
  border-left: 3px solid var(--color-danger);
  padding-left: 1rem;
}

/* ── Form submission loading state ── */
.form--loading .btn[type="submit"] {
  opacity: 0.6;
  pointer-events: none;
  position: relative;
}
.form--loading .btn[type="submit"]::after {
  content: '';
  position: absolute;
  right: 1rem; top: 50%;
  width: 1rem; height: 1rem;
  margin-top: -0.5rem;
  border: 2px solid rgba(255,255,255,0.3);
  border-top-color: white;
  border-radius: 50%;
  animation: spin 0.7s linear infinite;
}
@keyframes spin { to { transform: rotate(360deg); } }