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[mediaqueries] Queries for image support #656
Converting an old mail thread to a github issue for ease of tracking.
Recently, while browsing the web with image downloads disabled to save data, I was struck by how broken some sites were, despite being coded to current best practices. Screen reader-accessible text labels (for icon buttons) that were "visually hidden" with CSS were completely inaccessible to me. If I turned off those styles in the inspector, the site became usable again.
Wouldn't it be nice if a media query could restrict those style rules so that they aren't applied for users who don't receive the image icons?
Other use cases for changing styles based on whether or not images are supported:
Given that some browsers / accessibility modes support content images but not background images, the media query could distinguish between support for images in different contexts:
A second aspect of image support that it would often be useful to test for in a stylesheet is which types of images are supported. Currently, CSS has no way to test this support: a url() reference in a property value is valid even if the downloaded file type would be unrecognized. Newer high-compression image formats cannot be safely used with fallbacks to older file types unless you use JS-based support tests. In contrast, the HTML
Of course, if a browser does not currently support any images for this document, these should all evaluate as false. But the other media query is still useful for the general case and because it addresses the case when an image type is supported in content, but images are not downloaded at all for stylesheet properties. (Although I think that should also trigger an @supports test on the relevant CSS properties.)
For this kind of use cases, especially the first one, I think a pseudo class that matches on images that fail to load would be easier to use than a media query.
For this use case, it looks like using fallbacks in the
An element pseudoclass does have the benefit of applying to broken links as well as no-image settings. Mozilla already has a prefixed pseudoclass for this case. However, it puts constraints on which other elements you can effect with the selector (needs to be a direct sibling after the missing image in the DOM tree). This limits the use for doing things like un-hiding text labels and captions. Still better than nothing, of course.
Right. Maybe we could make something along these lines to work, and it would solve the problem:
Interesting, idea. Would be fun to generalize a :for(selector) pseudo-class for styling form element labels. Eg label:for(:required)
But doesn't address the too-common case where the image icon that failed to load is a CSS background image or generated content image.
I think that having a media query here would be useful
Also, your suggestion doesn't address the stylesheet
Also this may be related to whatwg/html#3631 if it is decided that appearance of image placeholders is to be controlled by CSS.
it seems that conditional rules that apply only if a particular type of resource is both supported and actually loaded is not specific to images. We have a very similar request for fonts in #2540. We have not resolved on a way to solve it yet, but it seems on first approximation that whatever we eventually do to solve that problem should solve both.
Maybe we should close this issue or that issue as a duplicate of the other, and broaden the scope of the one that isn't closed.
Maybe it would be better to be generalized to any elements with external resources.
I guess a browser or its extensions can, in many ways, disable/block any scripts, including inline scripts, in that case, I still would like to see a fallback mechanism to give users with some friendly hints about the current load status of various external resources, even including load failure of script files. Please refer to the issue @Malvoz referenced above.
I remembered about this discussion following a twitter thread asking how to test for WebP support. Tim Petrusky reports that having to use JPEG instead of WebP for background images increases download size by 50%. Bumping the thread to get some more attention on it.
For image format support, there are two ways we could handle it:
For "did this particular content image (or other element linking an external resource) fail to load/parse correctly", I think standardizing the Mozilla broken image pseudo-class is valid. We could also add a second pseudo for the "waiting to load" state, which becomes more important with native lazy loading.
For overall image / external resource download settings, I still think this makes the most sense as a media query. But at least some use cases could be handled by the other features above.