Mac: You already have a lot of different choices when it comes to Markdown editors. But if you're looking for something simple and built for web developers, MacDown is a free, open-source app that should fit the bill.
MacDown is based on Mou, which ceased development. Subsequently, it works the same in most ways. MacDown is fast, has a live preview mode, basic syntax highlighting, and is made to cater more to web developers than writers. It does its job well and is easy to pick up and start using if you've used Mou before.