Delayed images and delayed iframes can be used to enhance the performance of websites. With delayed images enabled, the page is sent without the image source URLs in the image elements. With delayed iframes enabled, the page is sent without the iframe source URLs in the iframe elements. This way the
load event is triggered earlier and the website is sooner responsive. The correct elements are loaded after the
load event is fired, or after the
ready event is fired for delayed images with
visibility setting and delayed iframes near the viewport.
To enable delayed images in the
<config> <acceleration> <image-delaying [ prioritization=" visibility" ] [ visibility-offset-x="disabled" ] [ visibility-offset-y="150" ] /> </acceleration> </config>
If delayed images are activated, all images are sent delayed by default, except for:
To enable delayed iframes in the
<config> <acceleration> <iframe-delaying [ visibility-offset-x="disabled" ] [ visibility-offset-y="150" ] /> </acceleration> </config>
If delayed iframes are activated, only iframes with an
ai-delay="true" attribute are sent delayed. Clients without
js support do not get delayed iframes.
Note: Using the delaying feature will add a
data-ai-hqsrc attribute to the
iframe elements that are delayed.
Note: Using this feature will add the class
AI_delayedHidden to the class list of all delayed elements to hide them initially. The class is removed as soon as each image and iframe has been loaded.
Note: When a document is requested via Partial Page Loading images will only be delayed in case the priority is set to
visibility. Delayed iframes are not affected by Partial Page Loading.
The order in which delayed images are loaded after the
load event occurred is determined by their
ai-priority attribute. Possible Values are
3 and the special value
3 being the earliest and
0 the latest. The default value is
If an image has an
ai-priority attribute with the value
visibility, it will start loading a certain amount of pixels before or after it enters the viewport, called the offset. Images that are initially visible in the viewport are loaded when the
ready event is fired.
ai-priority atribute is only applicable to delayed images. Delayed iframes are all using the visibility delaying.
disabledfor x-axis and
150for y-axis. Check Visibility Offsets for more information and options.
ai-priority= 0|1|2|3can be used to ensure images in a hidden container are loaded delayed but loading has finished before the container is shown.
<img src="logo.png" ai-priority="3"> <img src="image1.jpg" ai-priority="1"> <img src="image5.jpg"> <img src="image7.jpg" ai-priority="visibility">
The image with
ai-priority="3" is loaded before the image with
ai-priority="1". The image without
ai-priority attribute is loaded last. The image with
ai-priority="visibility" will be loaded when it enters the viewport.
The config setting
prioritization can override the priority of all delayed images with the value
visibility, regardless of
ai-priority attributes set in the source document. Allowed values are
auto. The value
visibility will deliver all images with the visibility setting,
auto delivers the images according to the
ai-priority attribute. The default is
prioritization=visibilitywill add an
ai-priorityattribute to all
imgelements that are delayed. Iframes will not get an additional attribute.
prioritization=visibilityis discouraged for use-cases where printing documents with delayed images is a regular task. Alternatively, a means must be provided to load all delayed images before printing (see
ai.images.loadHQImages()). Due to the asynchronous loading of the images the call to load delayed images must not be directly coupled with the call to print. The same applies to delayed iframes, which can be loaded with
image-delaying with visibility enabled and
iframe delaying you have two extended options:
visibility-offset-x (optional) (default:
visibility-offset-y (optional) (default:
Delayed elements are loaded if their top-left corner is within a certain ‘load’ area. The edges of this area are specified as offsets (in viewport pixels) to the visible viewport. If the horizontal visibility offset is positive, the area’s right edge is located to the right of the visible viewport’s right edge; if the offset is negative, its right edge is located to the left; if the offset is 0, both edges overlap. Similarly, if the vertical visibility offset is positive, the area’s bottom edge is located to the bottom of the visible viewport’s bottom edge; if the offset is negative, its bottom edge is located to the top; again, if the offset is 0, both edges overlap. If an offset has the value ‘disabled’, the area’s dimension in the respective direction is, so to speak, unlimited.
Example for 150 pixel vertical (y-axis) offset: Assuming a viewport height of 600 pixels, the ‘load’ area’s height is 750 pixels. A delayed element located 800px from the document’s top edge will not be loaded. When scrolling down 60 pixels, the element will cross the bottom edge of the ‘load’ area and will be loaded.
Example for -150 pixel horizontal (x-axis) offset: Assuming a viewport width of 400 pixels, the ‘load’ area’s width is 250 pixels. A delayed element located 300px from the document’s left edge will not be loaded. When scrolling to the right 60 pixels, the element will cross the right edge of the ‘load’ area and will be loaded.
Example for disabled horizontal (x-axis) offset: Assuming a viewport width of 400 pixels, the ‘load’ area’s width is unlimited. A delayed element will be loaded regardless of its left edge, provided that the element’s top edge is above the ‘load’ area’s bottom edge.
disabledignores scroll position for that axis
In order to avoid the “jumping page” effect, reduce reflows in the browser and increase the browser’s render performance, FIT delivers inline SVG placeholders with the same width and height as the delayed image, if
image-scalingfeature is enabled and
FIT automatically loads delayed images after page
load. However, if you insert delayed images by hand (for example using Fragments), you will have to initiate the loading yourself by calling the
Loads delayed images. Whether an image is loaded depends on its priority. If it is a number the image is loaded with a corresponding delay, if it is the value ‘visibility’ the image is loaded as soon as it enters the ‘load’ area.
trueif all delayed images should be loaded (ignoring their priority, or visibility in the ‘load’ area),
falseotherwise. The default is
false. This parameter does not affect the loading of non-visibility-delayed images.
FIT automatically loads delayed iframes after page
ready. However, if you insert delayed iframes by hand (for example using Fragments), you will have to initiate the loading yourself by calling the
Loads delayed iframes.
trueif all delayed iframes should be loaded (even if they are not in the viewport),
falseotherwise. The default is
hqimageloaded event is fired when a delayed image was loaded.
delayedimagesloaded event is fired after all delayed images have been loaded.
delayediframeloaded event is fired when a delayed iframe was loaded.
delayediframesloaded event is fired after all delayed iframes have been loaded.