Skip to content

Latest commit

 

History

History
244 lines (183 loc) · 11.7 KB

amp-html-layout.md

File metadata and controls

244 lines (183 loc) · 11.7 KB
Raw

AMP HTML Layout System

Overview

The main goal of the layout system is to ensure that AMP elements can express their layout so that the runtime is able to infer sizing of elements before any remote resources, such as JavaScript and data calls, have been completed. This is important since this significantly reduces rendering and scrolling jank.

With this in mind the AMP Layout System is designed to support few but flexible layouts that provide good performance guarantees. This system relies on a set of attributes such as layout, width, height, sizes and heights to express the element's layout and sizing needs.

Layout Types

AMP provides a set of layouts that AMP elements can use. They are defined using layout attributes described below. Not every element supports every layout - you have to refer to the element's documentation to know which layouts you can use.

nodisplay

The element is not displayed. This layout can be applied to every AMP element. It assumes that the element can display itself on user action, e.g. amp-lightbox.

fixed

The element has a fixed width and height with no responsiveness supported.

responsive

The element takes the space available to it and resizes its height automatically to the aspect ratio given by width and height attributes. This layout works very well for most of AMP elements, including amp-img, amp-video, etc.

The available space depends on the parent element and can also be customized using max-width CSS.

fixed-height

The element takes the space available to it but keeps the height unchanged. This layout works well for elements such as amp-carousel that involves content positioned horizontally.

fill

The element takes the space available to it - both width and height. In other words the layout of a fill element matches its parent.

container

The element lets its children to define its size, much like a normal HTML div.

Layout Attributes

width and height

Depending on the value of the layout attribute AMP component elements must have a width and height attribute that contains an integer pixel value. Actual layout behavior is determined by the layout attribute as described below.

In a few cases if width or height are not specified the AMP runtime can default these values as following:

  • amp-pixel: Both width and height are defaulted to 0.
  • amp-audio: The default width and height are inferred from browser.

layout

The optional layout attribute allows specifying how the component behaves in the document layout. Valid values for the layout attribute are:

  • Not present: The layout will be inferred as following:
    • if height is present and width is absent or equals to auto fixed-height layout is assumed;
    • if width or height attributes are present along with sizes and/or heights attribute, responsive layout is assumed;
    • if width or height attributes are present fixed layout is assumed;
    • if width and height are not present container layout is assumed
  • fixed: The width and height attributes must be present. The only exceptions are amp-pixel and amp-audio elements.
  • fixed-height: The height attribute must be present. The width attribute must not be present or must be equal to auto.
  • responsive: The width and height attributes must be present and are used to determine the aspect ratio of the component. The component is sized to the width of its container element while maintaining the height based on the aspect ratio.
  • fill: Element size will be determined by the parent element.
  • container: The component is assumed to not have specific layout itself but only act as a container. Its children are rendered immediately.
  • nodisplay: The component takes up zero space on the screen as if its display style was none. The width and height attributes are not required.

Each element documents which layout values it supported. If an element does not support the specified value it would trigger a runtime error.

sizes

All AMP custom elements that allow responsive layout, also support sizes attribute. The value of this attribute is a sizes expression as described in the img sizes, but extended to all elements, not just images. In short, sizes attribute describes how the width of the element is calculated depending on the media conditions.

When sizes attribute is specified along with width and height, the layout is defaulted to the responsive.

heights

All AMP custom elements that allow responsive layout, also support the heights attribute. The value of this attribute is a sizes expression based on media expressions as similar to the img sizes attribute, but with two key differences:

  1. It applies to the height and not width of the element.
  2. Percent values are allowed, e.g. 86%. If a percent value is used, it indicates the percent of the element's width.

When the heights attribute is specified along with width and height, the layout is defaulted to responsive.

An example:

<amp-img src="https://acme.org/image1.png"
    width="320" height="256"
    heights="(min-width:500px) 200px, 80%">
</amp-img>

In this example, the height of the element by default will be 80% of the width, but for the viewport wider than 500px it will be capped at 200px.

media

All AMP custom elements support the media attribute. The value of media is a media query. If the query does not match, the element is not rendered at all and it's resources and potentially it's child resources will not be fetched. If the browser window changes size or orientation the media queries are re-evaluated and elements are hidden and shown based on the new results.

Example: Here we have 2 images with mutually exclusive media queries. Depending on the screen width one or the other will be fetched and rendered. Note that the media attribute is available on all custom elements, so it can be used with non-image elements such as ads.

    <amp-img
        media="(min-width: 650px)"
        src="wide.jpg"
        width=466
        height=355 layout="responsive" ></amp-img>
    <amp-img
        media="(max-width: 649px)"
        src="narrow.jpg"
        width=527
        height=193 layout="responsive" ></amp-img>

placeholder

The placeholder attribute can be set on any HTML element, not just AMP elements. It indicates that the element marked with this attribute acts as a placeholder for the parent AMP element. If specified a placeholder element must be a direct child of the AMP element. By default, the placeholder is immediately shown for the AMP element, even if the AMP element's resources have not been downloaded or initialized. Once ready the AMP element typically hides its placeholder and shows the content. The exact behavior w.r.t. to placeholder is up to the element's implementation.

    <amp-anim src="animated.gif" width=466 height=355 layout="responsive" >
      <amp-img placeholder src="preview.png" layout="fill"></amp-img>
    </amp-anim>

fallback

The fallback attribute can be set on any HTML element, not just AMP elements. It's a convention that allows the element to communicate to the reader that the browser does not support it. If specified a fallback element must be a direct child of the AMP element. The exact behavior w.r.t. to fallback is up to the element's implementation.

    <amp-anim src="animated.gif" width=466 height=355 layout="responsive" >
      <div fallback>Cannot play animated images on this device.</div>
    </amp-anim>

noloading

Whether the "loading indicator" should be turned off for this element. Many AMP elements are whitelisted to show a "loading indicator", which is a basic animation that shows that the element has not yet fully loaded. The elements can opt out of this behavior by adding this attribute.

Behavior

A non-container (layout != container) AMP element starts up in the unresolved/unbuilt mode in which all of its children are hidden except for a placeholder (see placeholder attribute). The JavaScript and data payload necessary to fully construct the element may still be downloading and initializing, but the AMP runtime already knows how to size and layout the element only relying on CSS classes and layout, width, height and media attributes. In most cases a placeholder, if specified, is sized and positioned to take all of the element's space.

The placeholder is hidden as soon as the element is built and its first layout complete. At this point the element is expected to have all of its children properly built and positioned and ready to be displayed and accept a reader's input. This is the default behavior. Each element can override to, e.g., hide placeholder faster or keep it around longer.

The element is sized and displayed based on the layout, width, height and media attributes by the runtime. All of the layout rules are implemented via CSS internally. The element is said to "define size" if its size is inferrable via CSS styles and does not change based on its children: available immediately or inserted dynamically. This does not mean that this element's size cannot change. The layout could be fully responsive as is the case with responsive, fixed-height and fill layouts. It simply means that the size does not change without an explicit user action, e.g. during rendering or scrolling or post download.

If the element has been configured incorrectly it will not be rendered at all in PROD and in DEV mode the runtime will display the element in the error state. Possible errors include invalid or unsupported values of layout, width and height attributes.

(tl;dr) Appendix 1: Layout Table

What follows is the table of layouts, acceptable configuration parameters and CSS classes and styles used by this layouts. Notice that:

  1. Any CSS class marked prefixed with "-amp-" and elements prefixed with "i-amp-" are considered to be internal to AMP and their use in user stylesheets is not allowed. They are shown here simply for informational purposes.
  2. The only layouts that currently do not "define size" are container and nodisplay.
  3. Even though width and height are specified in the table as required the default rules may apply as is the case with amp-pixel and amp-audio.
Layout Width/Height Required? Defines Size? Additional Elements CSS "display"
nodisplay no no no none
fixed yes yes, specified by width and height no inline-block
responsive yes yes, based on parent container and aspect ratio of width:height yes, i-amp-sizer block
fixed-height height only. width can be auto yes, specified by the parent container and height no block
fill no yes, parent's size no block
container no no no block