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Clarify image.decode() failure on lazy-loaded images

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#38,034 opened on Feb 8, 2025

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Content:WebAPIarea: DOM/CSSOMhelp wanted

Description

MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/decode

What specific section or headline is this issue about?

Missing information

What information was incorrect, unhelpful, or incomplete?

As noted in https://stackoverflow.com/questions/65146920/domexception-invalid-image-request, the decode() function will throw EncodingError: Invalid image request. on images with loading="lazy" that have not yet loaded in Firefox; it will not do so in Chrome. This is either a browser bug, an indication that the page information ("This can be used to initiate loading of the image") is somewhat incomplete/misleading, or just of a case of undefined behavior which might deserve a note.

What did you expect to see?

Either just a note that this loading="lazy" case fails in Firefox on images that have not yet loaded, or an explanation of the difference between loading and decoding/when decoding might fail if this is actually a symptom of a larger degree of complexity.

edit: Per further testing, calling decode() in an onload handler works and functions correctly on a loading="lazy" image; it's only calling it too early that fails.

Do you have any supporting links, references, or citations?

The only semi-relevant Firefox bug I found is https://bugzilla.mozilla.org/show_bug.cgi?id=1758035, which links to the spec. It's still not entirely clear from the spec text to me whether the function is explicitly supposed to interact with/wait for loading, and if so exactly how.

Do you have anything more you want to share?

No response

MDN metadata

Contributor guide

Tech stack
javascripthtml
Domain
documentation
Issue type
documentation
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
HTMLImageElement APILazy loadingCross browser testing
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
80
Research direction
Review the existing documentation at files/en us/web/api/htmlimageelement/decode/index.md. Additionally, investigate the behavior described in the StackOverflow post and Firefox bug report (1758035) to confirm the current cross browser differences. Then update the page to note that calling decode() on an image with loading='lazy' before the image has loaded may throw an EncodingError in Firefox, and that using the onload event ensures proper behavior. Ensure the change aligns with MDN's documentation guidelines.