HTML writer: insert data- in front of unsupported attributes.

Thus, a span with attribute 'foo' gets written to HTML5
with 'data-foo', so it is valid HTML5.

HTML4 is not affected.

This will allow us to use custom attributes in pandoc without
producing invalid HTML.

1 files changed, 16 insertions, 0 deletions

diff --git a/test/command/custom-attributes.html b/test/command/custom-attributes.html
new file mode 100644
index 000000000..67dccc1b8
--- /dev/null
+++ b/test/command/custom-attributes.html
@@ -0,0 +1,16 @@
+Custom attributes should automatically have data- added
+in HTML5:
+```
+% pandoc -t html5
+[hello]{foo="bar"}
+^D
+<span data-foo="bar">hello</span>
+```
+
+but not in HTML4:
+```
+% pandoc -t html4
+[hello]{foo="bar"}
+^D
+<span foo="bar">hello</span>
+```