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amp-font
Description
Trigger and monitor the loading of custom fonts on AMP pages.
Required Scripts
<script async custom-element="amp-font" src="https://cdn.ampproject.org/v0/amp-font-0.1.js"></script>
Supported Layouts
font-display descriptor to specify how to display a font face dependent on whether and when it is ready. The amp-font component was AMP's solution to controlling timeouts on font loading. With the wide browser adoption of font-display, this component is no longer necessary. It is here for legacy users. Instead, use the font-display descriptor to specify how to display a font face dependent on whether and when it is ready.
Usage
Historically, the amp-font extension was used for controlling timeouts on font loading. We recommend implementing this functionality with font-display instead.
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 component observes loading of a font. When the font successfully loads it executes the optional on-load-add-class and on-load-remove-class attributes. If the font loading results in an error or timeout, it runs on-error-remove-class and on-error-add-class. These classes toggle on the documentElement for standalone documents, and on body for documents without a documentElement i.e. inside a ShadowRoot.
Use these classes to guard against displaying a font 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.
Attributes
font-family
The font family of the custom loaded font.
timeout (optional)
The allowed time in millisecond for the custom font to load. Defaults to 3000 when undefined. When set to 0 the component loads font only if the font is in the cache. If set to an invalid value it defaults back to 3000.
on-load-add-class (optional)
CSS class to add to document root after custom font is available.
on-load-remove-class (optional)
CSS class to remove from document root after custom font is available.
on-error-add-class (optional)
CSS class to add to the document root if the timeout interval runs out before the font is available.
on-error-remove-class (optional)
CSS class to remove from document root if the timeout interval runs out before the font is available.
font-weight (optional)
Behaves same as standard CSS font-weight property.
font-style (optional)
Behaves same as standard CSS font-style property.
font-variant (optional)
Behaves same as standard CSS font-variant property.
layout
The layout attribute must be set to nodisplay.
Validation
See amp-font rules in the AMP validator specification.
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