target="_blank" is a deprecated way to open a link in a new window. We try not to rely on this method as its not guaranteed to be supported in the future and we'd rather control behavior on the JavaScript level. 

If you look in https://github.com/edx/edx-platform/blob/master/cms/static/js/base.js for an example - linkNewWindow is the function we use and call through a JQuery selector ($('a[rel="external"]').attr('title', gettext('This link will open in a new browser window/tab')).bind('click', linkNewWindow);).