Here is a css file you can include via rawgit.com to get a reveiw comment style you can use in RMarkdown HTML output.
An example gist is here.
The icon comes courtesy of Nick Roach at the Elegant Themes Blog and is licensed GPLv2, as is the CSS file.
#Pdf/Html version
I'm currently experimenting with a function wrapper that can output to html and pdf interchangably. /r/rmd_comment.R will work, provided includes and headers are setup as per /test/pdf_html_test.Rmd
please ignore