Note, esp. for @ECohen_WMDE: It's possible to use CSS to style the very basic but rather unpredictable "broken image" experience. The possibilities include: Making the alt="…" text visible and style it, including size, weight, color, and shadow; background colors, gradients, repeated and non-repeated images. Think of e.g. an hourglass to indicate "something should have happened here". One aspect to consider is that this styling might be visible for a brief moment before the image successfully loads, notable esp. on slow bandwidth connections. The style shouldn't be aggressive because of this. Here are a few examples:

Something similar might be possible for missing tiles.

Next steps from sprint planning: make initial recommendations about potential improvements for each scenario. For example, whether we can show an error message or if there's some other bug to fix. Create new tasks for each group of improvements. Don't go very deep into proof-of-concept work without another round of discussion.

Missing features/broken map while loading (dynamic): Should be possible to add a background image with a spinner/whatever - TBD TICKET

I actually would argue against a spinner when a map or parts of it are loading in dynamic mode. I think it is very known to users that loading maps can take some time. Other map providers also do not display a spinner in those cases. Especially when we have multiple map tiles missing, displaying multiple spinners would be very challenging visually. 😵

@Lena_WMDE Ready for PM / UX review.

In addition to what I wrote in T305433#7920365 I played around with a little snippet of JavaScript that makes the existing error messages visible to the user. It will show up in place of the "This is the alt text …" messages in the example screenshots in T305433#7920365.

<style>
img.error {
	background: #f99;
}
</style>
<script>
function handleImgOnError( image ) {
	var request = new XMLHttpRequest();
	request.open( 'GET', image.src );
	request.send( null );
	request.onreadystatechange = function () {
		image.alt = request.responseText;
		image.className = 'error';
	}
}
</script>
<img src="thisWillFail.txt" width="400" height="400" onerror="handleImgOnError( this )">

At this point the error message can also be shown as a popup, in case we want that.