Knak has a templating tool that'll do something like this -- you can modify stuff on their platform and then import your work there into Marketo, they've been at it for a while and from what I hear it's pretty easy on the end-user. If you're using "just some email layout builder tool", there is a "Marketo layer" to think about if you go this route -- I wouldn't expect most tool to produce "Marketo-ready" html but they'd probably suffice to build out an email layout. One thing to be really aware of is that Marketo's Email 2.0 is module-based rather than "layout" or "template" based so the architecture of the template itself is more like a collection of modules than a "layout". Both the modules and the variables that make it super easy to update stuff have Marketo-specific syntax so this is something that you'd want to kind of lay over some HTML you made outside of Marketo. There are a few rules to check out in terms of the Container and Modules - specific elements and classes that need to be added for your email template to be valid. Here's a link to some of that syntax and examples that I keep bookmarked b/c it's a really useful resource for getting your custom HTML templates to gel with Marketo: Email Template Syntax - Marketo Docs - Product Documentation. If adding this code isn't something you're familiar with, it might still save you some time to spin up a design in whatever web tool and hand that off to a developer who should be able to wrap your code in some basic Marketo syntax pretty quickly which saves you some time on development.
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