Back when I taught corporate training classes, there would always be someone in my Microsoft Front Page or Adobe Dreamweaver class asking "Awwww, do I HAVE to learn HTML?"
I explain the relationship between HTML and front end design programs like this: If you come to me to take a class on Microsoft Word, I'm going to assume you know how to read and write. If you don't know how to read and write, learning Word will not help you!
Front end editors such as what you see in Marketo and tools like Front Page and Dreamweaver are fantastic and they do a lot of the grunt work for you, but if you don't really know what's going on behind the scenes you can get hurt when things don't work out the way you expect. It's even more important now that Marketo features like Guided Landing Page templates pretty much require HTML knowledge to use and customize them fully.
I have to be honest, back in the early 90s I didn't know that I wanted to learn HTML either. I had done a lot with computers by then, the world wide web was just getting started and people were talking about this "HTML thing" and my reaction was "I don't know, do I really want to learn one more thing?" Now, here I am writing tutorials!
So how do you get started? Way, way back in the day I picked up an excellent book called "The HTML Manual of Style" by Larry Aronson. The fourth edition appears to be the latest and greatest covering HTML 5 and CSS. I'm not sure of the rules for posting external links here, but you can find it at Amazon, Barnes and Noble and most good physical bookstores.
If you prefer a free way to get started, the one we recommend internally to Marketo staff is the W3Schools: