sed
If one would like to do this systematically for all external links, CSS is no option. However, one could run the following sed
command once the (X)HTML has been created from Markdown:
sed -i 's|href="http|target="_blank" href="http|g' index.html
This can be further automated in a single workflow when a Makefile
with build instructions is employed.
PS: This answer was written at a time when extension link_attributes
was not yet available in Pandoc.