brew install multimarkdown
brew install fswatch
echo '#!/bin/bash
multimarkdown --full "file.md" > /tmp/file.html && open -g /tmp/file.html' > /tmp/rebuild.sh && chmod +x /tmp/rebuild.sh; fswatch -o "file.md" | xargs -n1 /tmp/rebuild.sh
Safari reuses the same tab for opening a URL that is already open, so it will simply keep refreshing the same file.html tab every time you save changes to file.md. Who needs a dedicated markdown editor with an integrated preview window?