fix: Set alt attr on poster img

[mister-ben]

Description

As the poster now uses an img element, it should have an alt attribute.

Specific Changes proposed

Sets a null alt attribute on the poster image. This conveys to screen readers (and accessibility validators) that the image is decorative and does not need descriptive text.

To do in a future update (needs discussion)

• Allow an arbitrary string to be set, so that an image can have a meaningful alt text where appropriate. Requires update to poster/player options.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[OwenEdwards]

The <img> element should definitely have an alt attribute, but as far as whether it should have text or just a "null alt" (alt=""), that's a little complicated.

There was a lot of discussion at W3C about whether the <video> element should accept an alt attribute for the poster image, but the decision was that the poster image is an alternate for the video itself, so the title of the video (either as a title attribute or an aria-label/aria-labelledby on the video element) was effectively the text alternative for the image. That was under the specific expectation that the poster image is always a still from the video and not another image. In that case, I personally think the term "poster" is misleading because many people think of the movie theater poster when they hear that term related to a video. A movie theater poster is rarely just a still from the video and often includes text which isn't available to non-sighted users anywhere else. For example (from https://en.wikipedia.org/wiki/Gone_with_the_Wind_(film)):

So, I think the default should be to make it a null alt, but there should be an option to change that to a description of the poster image.

[mister-ben]

Thanks @OwenEdwards, very useful feedback. Let's keep this simple as a quick fix with the null alt and figure out how best to allow a more meaningful value to be set as a feature.

[mister-ben]

The codecov failures are codecov hitting rate limits. codecov/feedback#126. Apparently thr "fix" to add a codecov token to gh secrets doesn't apply to prs from forks, like this.

Well it runs successfully after hitting retry a few times (having to do that won't help their overall rate limiting issues) but the result is nonsense again, this PR definitely doesn't decrease coverage.

[codecov]

Codecov Report

Merging #8043 (c579a37) into main (e21d295) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #8043   +/-   ##
=======================================
  Coverage   81.91%   81.91%           
=======================================
  Files         110      110           
  Lines        7342     7342           
  Branches     1772     1772           
=======================================
  Hits         6014     6014           
  Misses       1328     1328           

Impacted Files	Coverage Δ
src/js/poster-image.js	75.60% <100.00%> (ø)

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