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amp-form

Description Allow usage of form and input tags.
Availability Experimental
Required Script <script async custom-element="amp-form" src="https://cdn.ampproject.org/v0/amp-form-0.1.js"></script>
Supported Layouts N/A
Examples Annotated code example for amp-form

Behavior

The amp-form extension allows the usage of forms and input fields in an AMP document. The extension allows polyfilling some of the missing behaviors in browsers.

The amp-form extension MUST be loaded if you're using <form> or any input tags, otherwise your document will be invalid!

Example:

<form method="post" action="https://example.com/subscribe" target="_blank">
    <fieldset>
        <label>
            <span>Your name</span>
            <input type="text" name="name" required>
        </label>
        <label>
            <span>Your email</span>
            <input type="email" name="email" required>
        </label>
        <input type="submit" value="Subscribe">
    </fieldset>
</form>

Attributes

target required

Target attribute of the <form> must be either _blank or _top.

action invalid when method=POST required when method=GET

Action must be provided, https and is non-cdn link (does NOT link to https://cdn.ampproject.org).

Note: target and action will only be used for non-xhr GET requests. AMP runtime will use action-xhr to make the request and will ignore action and target. When action-xhr is not provided AMP would make a GET request to action endpoint and use target to open a new window (if _blank). AMP runtime might also fallback to using action and target in cases where amp-form extension fails to load.

action-xhr required when method=POST optional when method=GET

You can also provide an action-xhr attribute, if provided, the form will be submitted in an XHR fashion.

An XHR request is where the browser would make the request without a full load of the page or opening a new page also sometimes called Ajax request. Browsers will send the request in the background using Fetch API when available and fallback to XMLHttpRequest API for older browsers.

This attribute can be the same or a different endpoint than action and has the same action requirements above.

Important: See Security Considerations for notes on how to secure your forms endpoints.

All other form attributes are optional.

custom-validation-reporting (optional) Enables and selects a custom validation reporting strategy, valid values are one of show-first-on-submit, show-all-on-submit or as-you-go.

See Custom Validation section for more details on this.

Inputs and Fields

Currently, <input type=button>, <input type=file>, <input type=image> and <input type=password> are not allowed.

Most of form-related attributes on inputs are not allowed, this include form, formaction, formtarget, formmethod and others.

(Relaxing some of these rules might be reconsidered in the future - please let us know if you require these and use cases).

Other input types, textarea, select, option, fieldset, label are allowed.

Events

amp-form exposes 3 events:

  • submit Emitted whenever the form is submitted and before the submission is complete.

  • submit-success Emitted whenever the form submission is done and response is a success.

  • submit-error Emitted whenever the form submission is done and response is an error.

These events can be used through the on attribute. For example, the following listens to both submit-success and submit-error and shows different lightboxes depending on the event.

<form ... on="submit-success:success-lightbox;submit-error:error-lightbox" ...>
</form>

See the full example here.

Success/Error Response Rendering

amp-form allows publishers to render the responses using Extended Templates.

Using submit-success and submit-error special marker attributes, publishers can mark any child element of form and include a <template></template> tag inside it to render the response in it.

The response is expected to be a valid JSON Object. For example, if the publisher's action-xhr endpoint returns the following responses:

Success Response

{
  "name": "Jane Miller",
  "interests": [{"name": "Basketball"}, {"name": "Swimming"}, {"name": "Reading"}],
  "email": "email@example.com"
}

Error Response

{
  "name": "Jane Miller",
  "message": "The email (email@example.com) you used is already subscribed."
}

Publishers can render these in a template inside their forms as follows.

<form ...>
    <fieldset>
      ...
    </fieldset>
    <div submit-success>
        <template type="amp-mustache">
            Success! Thanks {{name}} for subscribing! Please make sure to check your email {{email}}
            to confirm! After that we'll start sending you weekly articles on {{#interests}}<b>{{name}}</b> {{/interests}}.
        </template>
    </div>
    <div submit-error>
        <template type="amp-mustache">
            Oops! {{name}}, {{message}}.
        </template>
    </div>
</form>

See the full example here.

Polyfills

amp-form provide polyfills for behaviors and functionality missing from some browsers or being implemented in the next version of CSS.

Invalid Submit Blocking and Validation Message Bubble

Browsers that uses webkit-based engines currently (as of August 2016) do not support invalid form submissions. These include Safari on all platforms, and all iOS browsers. amp-form polyfills this behavior to block any invalid submissions and show validation message bubbles on invalid inputs.

Note: Messages are sometimes limited to a few words like "required field", these messages are provided by the browser implementation. We'll be working on allowing publisher-provided custom validation messages as well as custom validation UIs (instead of the builtin/polyfill'd bubbles).

:user-invalid/:user-valid

These pseudo classes are part of the future CSS Selectors 4 spec and are introduced to allow better hooks for styling invalid/valid fields based on a few criteria.

One of the main differences between :invalid and :user-invalid is when are they applied to the element. :user-invalid is applied after a significant interaction from the user with the field (e.g. user types in a field, or blur from the field).

amp-form provides classes (see below) to polyfill these pseudo-classes. amp-form also propagates these to ancestors fieldsets and form.

Classes and CSS Hooks

amp-form provides classes and CSS hooks for publishers to style their forms and inputs.

.amp-form-submitting, .amp-form-submit-success and .amp-form-submit-error are added to indicate the state of the form submission.

.user-valid and .user-invalid classes are a polyfill for the pseudo classes as described above. Publishers can use these to style their inputs and fieldsets to be responsive to user actions (e.g. highlighting an invalid input with a red border after user blurs from it).

See the full example here on using these.

Custom Validations

amp-form provides a way for you to build your own custom validation UI with few validation reporting strategies available to choose from show-first-on-submit, show-all-on-submit or as-you-go.

The general usage of this is you first set custom-validation-reporting attribute on your form to one of the validation reporting strategies and then provide your own validation UI marked up with special attributes, AMP will discover these and report them at the right time depending on the strategy selected.

Here's an example (for more examples please check examples/forms.amp.html):

<h4>Show All Invalid Messages On Submit</h4>
<form method="post"
      action-xhr="/form/echo-json/post"
      target="_blank"
      custom-validation-reporting="show-all-on-submit">
    <fieldset>
        <label>
            <span>Your name</span>
            <input type="text" name="name" id="name5" required pattern="\w+\s\w+">
            <span visible-when-invalid="valueMissing" validation-for="name5"></span>
            <span visible-when-invalid="patternMismatch" validation-for="name5">
                Please enter your first and last name separated by a space (e.g. Jane Miller)
            </span>
        </label>
        <label>
            <span>Your email</span>
            <input type="email" name="email" id="email5" required>
            <span visible-when-invalid="valueMissing" validation-for="email5"></span>
            <span visible-when-invalid="typeMismatch" validation-for="email5"></span>
        </label>
        <input type="submit" value="Subscribe">
    </fieldset>
</form>

For validation messages, if your element contains no text content inside, AMP will fill it out with the browser's default validation message. In the example above, when name5 input is empty and validation kicked off (i.e. user tried to submit the form) AMP will fill <span visible-when-invalid="valueMissing" validation-for="name5"></span> with the browser validation message and show that span to the user.

Reporting Strategies

Show First on Submit

This mimics the browser default behavior when default validation kicks in. It shows the first validation error it finds and stops there.

Show All on Submit

This shows all validation errors on all invalid inputs when the form is submitted. This is useful if you'd like to show a summary of validations for example.

As You Go

This allows your user to see validation messages as they're interacting with the input, if the email they typed is invalid they'll see the error right away and once fixed the error goes away.

Security Considerations

Your XHR endpoints need to follow and implement CORS Requests in AMP spec.

Protecting against XSRF

In addition to following AMP CORS spec, please pay extra attention to state changing requests note.

In general, keep in mind the following points when accepting input from the user:

  • Only use POST for state changing requests.
  • Use non-XHR GET for navigational purposes only, e.g. Search.
    • non-XHR GET requests are not going to receive accurate origin/headers and backends won't be able to protect against XSRF with the above mechanism.
    • In general use XHR/non-XHR GET requests for navigational or information retrieval only.
  • non-XHR POST requests are not allowed in AMP documents. This is due to inconsistencies of setting Origin header on these requests across browsers. And the complications supporting it would introduce in protecting against XSRF. This might be reconsidered and introduced later, please file an issue if you think this is needed.