Get cron to send html-formatted emails
I don't think you can. A solution could be to pipe the output to sendmail or mail, which gives you more control over content type and other headers.
I put the CONTENT_TYPE="text/plain; charset=utf-8" in /etc/crontab to have my mail in UTF-8.
Put CONTENT_TYPE="text/html; charset=utf-8" in /etc/crontab to send the mail in HTML.
You may add it in specific file if the directory /etc/cron.d/ exists. In this case, the specification is defined only for the requested tasks.