Remove the rel and rev attributes from footnotes

Since the `rev` attribute has been removed from the HTML 5 spec and
the `rel` attribute doesn't accept `"footnote"` as a value anymore,
let's remove them from the generated output for footnotes.

It's still possible to have the previous behaviour with a custom
render object.

Fixes #536.

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+* Remove the `rel` and `rev` attributes from the output generated
+  for footnotes as they don't pass the HTML 5 validation.
+
+  Fixes #536.
+
 * Automatically enable the `fenced_code_blocks` option passing a
   `HTML_TOC` object to the `Markdown` object's constructor since
   some languages rely on the sharp to comment code.

ext/redcarpet/html.c

@@ -656,7 +656,7 @@ rndr_footnote_def(struct buf *ob, const struct buf *text, unsigned int num, void
 	bufprintf(ob, "\n<li id=\"fn%d\">\n", num);
 	if (pfound) {
 		bufput(ob, text->data, i);
-		bufprintf(ob, "&nbsp;<a href=\"#fnref%d\" rev=\"footnote\">&#8617;</a>", num);
+		bufprintf(ob, "&nbsp;<a href=\"#fnref%d\">&#8617;</a>", num);
 		bufput(ob, text->data + i, text->size - i);
 	} else if (text) {
 		bufput(ob, text->data, text->size);
@@ -667,7 +667,7 @@ rndr_footnote_def(struct buf *ob, const struct buf *text, unsigned int num, void
 static int
 rndr_footnote_ref(struct buf *ob, unsigned int num, void *opaque)
 {
-	bufprintf(ob, "<sup id=\"fnref%d\"><a href=\"#fn%d\" rel=\"footnote\">%d</a></sup>", num, num, num);
+	bufprintf(ob, "<sup id=\"fnref%d\"><a href=\"#fn%d\">%d</a></sup>", num, num, num);
 	return 1;
 }
 

test/html_render_test.rb

@@ -140,14 +140,14 @@ def test_that_footnotes_work
     Markdown
 
     html = <<-HTML.chomp.strip_heredoc
-      <p>This is a footnote.<sup id="fnref1"><a href="#fn1" rel="footnote">1</a></sup></p>
+      <p>This is a footnote.<sup id="fnref1"><a href="#fn1">1</a></sup></p>
 
       <div class="footnotes">
       <hr>
       <ol>
 
       <li id="fn1">
-      <p>It provides additional information.&nbsp;<a href="#fnref1" rev="footnote">&#8617;</a></p>
+      <p>It provides additional information.&nbsp;<a href="#fnref1">&#8617;</a></p>
       </li>
 
       </ol>