The PR yihui/knitr#2243 has now been merged.

@dmurdoch have you considered having {knitr} automatically add the aria-labelledby attribute to the widget's HTML (by extracting the id from the HTML and using that to populate the aria-labelledby attribute)? That seems like it could lead to a cleaner solution than #469 (it doesn't feel quite right for {htmlwidgets} to provide options for a {knitr} feature). The main thing I'm not sure of that might be an issue is whether the aria-labelledby attribute is ever not desirable when fig.cap is provided (but even in that case, it still feels like {knitr}'s responsibility to add an opt-out for that?)

@cpsievert : I hadn't considered that before. Now I have, I think it's a bad idea: currently knitr doesn't touch the code of an htmlwidget that it's including in a document. To do so, it would need to parse enough of the htmlwidget code to know that it's safe to modify it. I don't think knitr currently does anything like that.

As to the last part of your question: I can imagine a widget that provides an alternate label that is unrelated to fig.cap/fig.alt from knitr, and does it in a way that knitr wouldn't recognize. Then there'd be confusion.

So I think the current solution is better (though obviously there might be an even better one).

To do so, it would need to parse enough of the htmlwidget code to know that it's safe to modify it. I don't think knitr currently does anything like that.

That's a fair point. Perhaps a workaround would be to get/set attributes via JS? Maybe by having knitr:::add_html_caption() append a <script> tag that looks something like this?

<script>
(function() {
  var el = document.getElementById({{id}}); // id would be populated in R via the widget's HTML
  var aria = el.getAttribute('aria-labelledby');
  if (!aria) {
     el.setAttribute('aria-labelledby', {{id}} + "-knitr-aria");
     el.parentElement.querySelector('.caption').setAttribute('id', {{id}} + "-knitr-aria");
  }
  document.currentScript.remove();
})();
</script>

The Javascript would need to be more complicated than that - the widget would need some way to communicate to knitr that it wants the label. That's what my htmlwidgets PR was doing with the aria-labelledby attribute. Some widgets shouldn't get aria labels -- see @gtritchie's first comment to the PR #469.