Minor fixed in .htaccess
https://html5sec.org/r/data (no trailing slash)
I know, strangely your regex did not work on my box, I blame it on a version-based behavior difference. The website H5SC is hosted on is a small VPS so everything is a bit orthodox version-wise.
For reference I was running: Apache 2.2.22-14
The H5SC is on a Apache 2.2.14 - maybe that's the reason?