Right now, you have to be somewhat proficient in CSS and HTML to use the design manager for marketing assets such as landing pages and emails. This backend design platform is not very user-friendly and is somewhat confusing to use.
I come from a background of frontend design platforms like MailChimp for emails, and Instapage for landing pages. Those platforms were so easy to use and the marketing assets I was able to create were so clean and professional, all because they made it easy and user-friendly for all designers and marketing professionals (all levels, beginner to advance).
You can drag and drop elements (picture and text blocks) right onto the design space and see what it loos like right then and there. I feel that HubSpot should really consider a frontend design platform.