amp-font

Trigger and monitor the loading of custom fonts on AMP pages.

Required Script	`<script async custom-element="amp-font" src="https://cdn.ampproject.org/v0/amp-font-0.1.js"></script>`
Supported Layouts	nodisplay
Examples	Annotated code example for amp-font

Behavior

The amp-font extension should be used for controlling timeouts on font loading.

The amp-font extension allows adding and removing CSS classes from document.documentElement or document.body based on whether a font was loaded or is in error-state.

Example:

  <amp-font
      layout="nodisplay"
      font-family="My Font"
      timeout="3000"
      on-error-remove-class="my-font-loading"
      on-error-add-class="my-font-missing"></amp-font>
  <amp-font
      layout="nodisplay"
      font-family="My Other Font"
      timeout="1000"
      on-load-add-class="my-other-font-loaded"
      on-load-remove-class="my-other-font-loading"></amp-font>

The extension observes loading of a font and when it loads executes the optional attributes on-load-add-class and on-load-remove-class and when there is any error or timeout runs on-error-remove-class and on-error-add-class. These classes are toggled on the documentElement for standalone documents, and on body for documents without a documentElement i.e. inside a ShadowRoot.

Using these classes authors can guard whether a font is displayed and get the following results:

get a short (e.g. 3000ms) timeout in Safari similar to other browsers
implement FOIT where the page renders with no text before the font comes in
make the timeout very short and only use a font if it was already cached.

The amp-font extension accepts the layout value: nodisplay

Attributes

font-family	The font-family of the custom font being loaded.
timeout	Time in milliseconds after which the we don't wait for the custom font to be available. This attribute is optional and it's default value is 3000. If the timeout is set to 0 then the amp-font loads the font if it is already in the cache, otherwise the font would not be loaded. If the timeout is has an invalid value then the timeout defaults to 3000.
on-load-add-class	CSS class that would be added to the document root after making sure that the custom font is available for display. This attribute is optional.
on-load-remove-class	CSS class that would be removed from the document root after making sure that the custom font is available for display. This attribute is optional.
on-error-add-class	CSS class that would be added to the document root if the timeout interval runs out before the font becomes available for use. This attribute is optional.
on-error-remove-class	CSS class that would be removed from the document root if the timeout interval runs out before the font becomes available for use. This attribute is optional.
font-weight, font-style, font-variant	The attributes above should all behave like they do on standard elements.
layout	Must be `nodisplay`.

Validation

See amp-font rules in the AMP validator specification.

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