This also comes from the Litmus Survey 2018 and it does not take brains to guess that the question was about an email framework in use. The biggest advantages in using frameworks is that they have already solved for you most of the quirks of email clients. They also provide you with an established workflow you can follow, both alone and as a team. They also handle responsive design very well, albeit differently from one another.
Bee Pro
Cerberus
HEML
HTML Email Boilerplate
Litmus Community Templates
MailChimp’s Blueprints
Mailjet’s MJML
Zurb’s Foundation for Emails
*The one provided by our email service provider*