We did a trial of Exact Target Content Builder a while back. It's definitely not a fool-proof system, but I was pleased by the features provided for content organization and templatization of emails. We also wanted to get to a place where marketing folks could start putting together their own emails. My strategy would be to hand-code self-contained modular blocks that can exist by themselves or be stacked together to form a larger email. That way you can test them individually and then just stack them like LEGOs. While the WYSIWYG editor was pretty versatile in what it allowed you to build by just dropping in images and text, the resulting content blocks were not responsive when I tested them.
I found this in another discussion thread. Using the zero-width non-joiner and non-breaking space characters one after another will push out unwanted preview text. Each pair will create one blank space, so you may have to use a sizable block depending on how much empty space you want to create.
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This is not a class but a super helpful technique that allows you to make responsive containers that can resize when they go mobile. https://medium.freecodecamp.com/the-fab-four-technique-to-create-responsive-emails-without-media-queries-baf11fdfa848#.4ctq77p3u