HTML Email sounds easy, but it is actually one of the most difficult things to get right. There are so many different email clients, all of which treat HTML slightly differently. Unfortunately it's back to coding like it's 1990, with tables and inline styling. But it's worth it to see your emails work perfectly everywhere. I started my journey in email, and I still code them regularly for clients, so I have good knowledge of what will and won't work and the fixes that need to be implemented to make sure that the code will work and render perfectly everywhere. I have a high standard when it comes to coding, and email HTML is a skill that requires this perfectionist attitude to be successful.
Responsive emails became popular a few years ago, and I quickly developed a responsive framework which is now used across the company where I started. I have continued to add to this framework and develop documentation around it and I've made it publicly available for use.