Support aria-labelledby in htmlwidgets.

NEWS.md

@@ -6,6 +6,8 @@
 
 - The global option `knitr.progress.simple` can be used to decide whether to output the bar in the progress. When set to `FALSE`, only the step numbers and chunk labels will be printed, and the progress bar is omitted. This can be more useful for logging purposes since the bar itself is not useful (thanks, @hadley, #2221). By default, the simple progress output is used when the progress is not written to a connection such as `stdout` or `stderr` (e.g., written to a file instead), or the output connection is not a "terminal".
 
+- HTML Widgets can now support alt text by specifying an attribute `aria-labelledby="<label>"` in their first HTML tag. The text will be obtained from the `fig.alt` or `fig.cap` chunk option in the usual way (thanks, @dmurdoch, #2243).
+
 ## BUG FIXES
 
 - The chunk option `collapse = TRUE` works with HTML widgets now (thanks, @dmurdoch, #2212).

R/output.R

@@ -499,7 +499,12 @@ sew.knit_asis = function(x, options, inline = FALSE, ...) {
     if (inherits(x, 'knit_asis_htmlwidget')) {
       options$fig.cur = plot_counter()
       options = reduce_plot_opts(options)
-      return(add_html_caption(options, wrap_asis(x, options)))
+      # look for attribute 'aria-labelledby="label"' in the first HTML tag and
+      # use the label to provide alt text if found
+      return(add_html_caption(
+        options, wrap_asis(x, options),
+        xfun::grep_sub('^[^<]*<[^>]+aria-labelledby[ ]*=[ ]*"([^"]+)".*$', '\\1', x)
+      ))
     }
   }
   x = wrap_asis(x, options)
@@ -642,12 +647,27 @@ sew.knit_embed_url = function(x, options = opts_chunk$get(), inline = FALSE, ...
   ))
 }
 
-add_html_caption = function(options, code) {
+add_html_caption = function(options, code, id = NULL) {
   cap = .img.cap(options)
-  if (cap == '') return(code)
+  if (cap == '' && !length(id)) return(code)
+
+  if (length(id)) {
+    alt = .img.cap(options, alt = TRUE)
+    if (cap == alt && cap != '') {
+      # both are the same, so insert cap with id
+      alttext = sprintf('<p class="caption" id="%s">%s</p>\n', id, cap)
+      # prevent a second insertion
+      cap = ''
+    } else {
+      alttext = sprintf('<p id="%s" hidden>%s</p>\n', id, alt)
+    }
+  } else alttext = ''
+
+  captext = if (cap == '') '' else sprintf('<p class="caption">%s</p>\n', cap)
+
   sprintf(
-    '<div class="figure"%s>\n%s\n<p class="caption">%s</p>\n</div>',
-    css_text_align(options$fig.align), code, cap
+    '<div class="figure"%s>\n%s\n%s%s</div>',
+    css_text_align(options$fig.align), code, captext, alttext
   )
 }