Update fallback content's behavior for ImageInputType and HTMLImageElement

Category: DOM
Type: New or changed feature
Status: Enabled by default (Chrome 85)
Intent stage: Shipped

Summary

Previously, there have been inconsistencies on the implementation of fallback content and its treatment as a replaced element for ImageInputType and HTMLImageElement. This led to interoperable issues between implementors. Examples: <input style="display: inline;" type=image alt="ALT"> <img title="foo"> <img src="foo" width=10 height=10 alt> Please see, https://github.com/yuzhe-han/images-fallback-content-1094763, for visual comparison.

Motivation

The behavior of ImageInputType's and HTMLImageElement's fallback content was not very interoperable, and not clearly specified. With this HTML spec PR (https://github.com/whatwg/html/pull/5315) the behavior has been more completely specified. Chrome will implement this modified behavior starting in M85. See https://github.com/whatwg/html/issues/5309, https://bugzilla.mozilla.org/show_bug.cgi?id=1196668, and tracking bugs for more details and conversation.

Standards & signals

Specification: https://github.com/whatwg/html/pull/5315/
Firefox: Shipped/Shipping
Safari: Shipped/Shipping
Web developers: No signals
Tracking bug: https://crbug.com/1094763

Docs: N/A

Explainers: https://github.com/whatwg/html/issues/5309

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