Improve the Gallery images alt attribute within the editor block

[afercia]

Describe the bug
Gallery images in Gallery block within the editor are typically rendered with the following markup (new lines are for better readability in this example):

<img
	src=" ... "
	alt="Huatulco Coastline"
	data-id="770"
	tabindex="0"
	aria-label="image 3 of 4 in gallery"
>

The aria-label="image 1 of 4 in gallery" overrides the alt attribute, making the images not recognizable by screen reader users. The only available information is image 3 of 4 in gallery, which doesn't help that much.

To reproduce
Steps to reproduce the behavior:

• create a post
• insert a gallery containing images with alt attributes
• check the source in your browser's dev tools
• see both the alt attribute and the aria-label attribute are present
• optionally: try to use a screen reader and navigate to a gallery image within the editor (that's not easy)
• verify the images are announced only with the aria-label content, e.g. image 3 of 4 in gallery

Expected behavior
The alt attribute to not be overridden

Desktop (please complete the following information):
N/A

Smartphone (please complete the following information):
N/A

Possible solution:

• Prepend the alt attribute (if any) to the aria-label
• To be verified: whether the info image 3 of 4 in gallery is really necessary: gallery images are within list item and the number of items should be automatically announced by screen readers

[paaljoachim]

@glendaviesnz Glen

As your working on #25940
I do believe this is an issue you might want to know about.

[glendaviesnz]

With the merging of the gallery block refactor the gallery images are now Image blocks, which don't currently have an aria label so alt tag should be read by default.