decoding

The decoding attribute, valid on <image> elements, provides a hint to the browser as to whether it should perform image decoding along with rendering other content in a single presentation step that looks more "correct" (sync), or render and present the other content first and then decode the image and present it later (async). In practice, async means that the next paint does not wait for the image to decode.

It is often difficult to perceive any noticeable effect when using decoding on static <image> elements. They'll likely be initially rendered as empty images while the image files are fetched (either from the network or from the cache) and then handled independently anyway, so the "syncing" of content updates is less apparent. However, the blocking of rendering while decoding happens, while often quite small, can be measured — even if it is difficult to observe with the human eye.

Using different decoding types can result in more noticeable differences when dynamically inserting <image> elements into the DOM via JavaScript — see SVGImageElement.decoding for more details.

Allowed values:

sync

Decode the image synchronously along with rendering the other content, and present everything together.

async

Decode the image asynchronously, after rendering and presenting the other content.

auto

No preference for the decoding mode; the browser decides what is best for the user. This is the default value.