Accessibility support

[dmurdoch]

By default, htmlwidgets are wrapped in <div> tags, which don't support "alt" attributes for explanatory text. In https://github.com/yihui/knitr/pull/2243 I have proposed that htmlwidgets which use the "aria-labelledby" attribute should get a text label added, which (at least some) screen readers will treat like alt text.

I can do this for the rgl widget by adding a widget_html.rglWebGL() function (https://github.com/dmurdoch/rgl/blob/635349d8c2c7deef5fcfa292340c93fb254a3286/R/rglwidget.R#L401-L404) but maybe this should be added by the htmlwidgets:::widget_html.default() function (https://github.com/ramnathv/htmlwidgets/blob/e234c0e88ad359c7f4d35c52f9da9a3d85a392ae/R/htmlwidgets.R#L287-L293), probably conditional on some setting in the widget.

[dmurdoch]

The PR https://github.com/yihui/knitr/pull/2243 has now been merged.

[cpsievert]

@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?)

[dmurdoch]

@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).

[cpsievert]

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>

[dmurdoch]

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 https://github.com/ramnathv/htmlwidgets/pull/469.