---
name: form-design
description: Helper text, placeholders, validation and submit behaviour. Use when designing or reviewing a form, input field or data entry UI.
metadata:
  priority: 8
  pathPatterns:
    - "components/**"
    - "src/components/**"
    - "**/*.tsx"
    - "**/*.jsx"
    - "**/*.html"
    - "design-system/**"
    - "ui/**"
  promptSignals:
    phrases:
      - "form"
      - "input"
      - "validation"
      - "placeholder"
      - "helper text"
      - "field"
      - "submit"
      - "required"
      - "error message"
      - "fieldset"
retrieval:
  aliases:
    - form design
    - input validation
    - helper text
    - placeholder
    - form validation
    - real-time validation
    - submit button state
  intents:
    - design a form
    - add validation to inputs
    - write helper text
    - enable submit when valid
    - add real-time validation
    - improve form usability
  examples:
    - add helper text to this input
    - validate this field in real time
    - disable the submit button until the form is valid
    - design this form with proper validation
---

# Form Design

Forms are where users give the product data. Every unnecessary obstacle between the user and a completed form is a failure. The design goal is to make correct input easy and incorrect input obvious — before the user submits.

---

## The Three Guidance Layers

Each layer serves a distinct purpose. Do not collapse them.

### Layer 1 — Helper Text
Explains *what* to enter. Appears below the input, always visible, in small secondary text.

```
Email address
[                              ]
Use the email you signed up with.
```

- Write in plain language from the user's perspective
- Keep it to one sentence — if you need more, the field is too complex or misnamed
- Do not repeat the label ("Enter your email" below a label that says "Email" is redundant)
- Helper text is not a replacement for a label — the label is still required

### Layer 2 — Placeholder
Shows the *format* or an example value. Appears inside the input, disappears on typing.

```
[jane@example.com              ]
```

- Use a realistic example, not a description: `+358 40 123 4567` not `Enter phone number`
- Never use placeholder as a label — it disappears and leaves the user without context
- Keep it grey (`--color-text-secondary`) and lighter than actual input text
- Optional — not every field needs a placeholder

### Layer 3 — Validation
Confirms whether the input is correct. The most important layer.

```
Email address
[jane@           ] ← invalid
✗ Enter a valid email address.
```

**Validation timing:**
- **On blur** (leaving the field): default for most fields — validates once the user has finished
- **Real-time** (on input): use when the format is complex or the error is likely — password strength, IBAN, VAT number, URL, regex-heavy fields
- **On submit**: catches anything missed, scrolls to the first error

Real-time validation must be forgiving at the start — do not show an error the instant the user starts typing. Show it after a short debounce (300–500ms) or after the first character that makes the input definitively wrong.

---

## Submit Button State

The submit button enables when the form is valid. This is one of the clearest affordance signals in form design — the user sees the goal and knows when they have reached it.

```
[Submit]   ← disabled, low contrast, cursor: not-allowed
           (fields incomplete or invalid)

[Submit]   ← enabled, full colour, cursor: pointer
           (all required fields valid)
```

**Implementation:**
```html
<button type="submit" disabled={!isFormValid}>Submit</button>
```

For long or complex forms where real-time validation is not practical, do not disable the submit — validate on submit and scroll to errors instead. Disabled submit on a long form frustrates users who cannot tell what is missing.

**Loading state on submit:** Replace label with spinner, disable the button. Prevent double-submission.

---

## Field Anatomy

```
[Label]                           [Optional badge if optional]
[Input field                                                  ]
[Helper text — what to enter, format, constraints            ]
[Error message — appears below helper text on validation fail ]
```

```html
<div class="field">
  <label for="vat">VAT number <span class="optional">Optional</span></label>
  <input
    id="vat"
    type="text"
    placeholder="FI12345678"
    aria-describedby="vat-helper vat-error"
    aria-invalid="true"
  >
  <p id="vat-helper" class="helper-text">Finnish VAT numbers start with FI followed by 8 digits.</p>
  <p id="vat-error" class="error-text" role="alert">Enter a valid Finnish VAT number (e.g. FI12345678).</p>
</div>
```

---

## Required vs Optional

Mark the minority. If most fields are required, mark the optional ones. If most are optional, mark the required ones.

- Do not rely on colour alone — add a text label ("Required" or asterisk with legend)
- Place the required/optional indicator in the label, not only in the placeholder or helper text

```html
<label>Email <abbr title="Required">*</abbr></label>
<!-- or -->
<label>Phone <span class="badge">Optional</span></label>
```

---

## Grouping with Fieldset

Related fields belong in a `<fieldset>` with a `<legend>`. This is semantic HTML and helps screen readers announce the group context.

```html
<fieldset>
  <legend>Billing address</legend>
  <label>Street</label><input type="text">
  <label>City</label><input type="text">
  <label>Postal code</label><input type="text">
</fieldset>
```

Use fieldsets for:
- Address groups
- Payment details
- Radio button groups
- Checkbox groups

---

## Input Types

Use the correct `type` — browsers provide free validation, appropriate keyboards, and autofill.

| Data | Input type |
|---|---|
| Email | `type="email"` |
| Phone | `type="tel"` |
| URL | `type="url"` |
| Number | `type="number"` |
| Password | `type="password"` |
| Date | `type="date"` |
| Search | `type="search"` |
| Colour | `type="color"` |

On mobile, `type="email"` shows the email keyboard, `type="tel"` shows the numpad. These are free UX improvements.

---

## Autofill Support

Allow browsers to autofill. Do not disable it unless there is a security requirement.

```html
<input type="text"  autocomplete="name">
<input type="email" autocomplete="email">
<input type="tel"   autocomplete="tel">
<input type="text"  autocomplete="street-address">
<input type="text"  autocomplete="postal-code">
<input type="text"  autocomplete="cc-number">    <!-- credit card -->
<input type="password" autocomplete="new-password">
```

Correct `autocomplete` values reduce friction dramatically for returning users and on mobile.

---

## Copy That Depends on a Count

Any string whose wording depends on a number goes through the message catalogue, with the count as a named placeholder. Never branch on `count === 1` in the view. That ternary hard-codes three English facts: two plural forms, the split at one, the number before the noun. Finnish inflects the noun, other languages have three or more forms or trail the count. The bug is invisible until translation, then spread across every view.

Translating UI copy makes every text-based test selector locale-dependent. Pin the locale in the test setup in the same change.

---

## Review Checklist

- [ ] Every field has a visible label (not just placeholder)
- [ ] Helper text is below the input and explains what to enter
- [ ] Placeholder shows format or example, not a description
- [ ] Validation triggers on blur for simple fields, real-time for complex ones
- [ ] Error message is adjacent to the field that failed
- [ ] Error message is associated via `aria-describedby`
- [ ] Required/optional marked on the minority of fields
- [ ] Submit button is disabled when form is invalid (for short forms)
- [ ] Submit button shows a loading state and prevents re-submission
- [ ] Related fields are grouped in `<fieldset>` with `<legend>`
- [ ] Correct `type` attribute on all inputs
- [ ] `autocomplete` attributes set on address, contact, and payment fields
- [ ] Count-dependent copy resolved through the catalogue with the count as a placeholder, never a one/other ternary
- [ ] Tests that select by text pin the locale