Don't know if this is exactly what you're looking for, but in Litmus you can see the rendered code for some email clients through the interface in Builder with the 'view processed html'.
Another option is to open the preview image in Litmus checklist/builder in a new tab and change the .png extension to .html. This works for some clients and this way you can see the rendered html in the browser as well.
You could also copy and paste the content of an email to http://whatdoesitpaste.com/. This gives you an idea on how the html looks like in for example Outlook for Mac.
Maybe this will help you in the right direction.
Hi Mark, I did some testing a while ago and these were the results:
Outlook 2016/2019 for Mac
iOS Native Mail App
Samsung Native Mail App
Outlook App Android en iOS
Not supported in:
Outlook 2007 and higher on Windows
Gmail App Android en iOS**
Browser support in Chrome, Safari and Firefox, not IE/EDGE. source: https://caniuse.com/#feat=apng
* Supported when using outlook.com account
Another great resource on this and images in email in general: https://codepen.io/M_J_Robbins/pen/PKXaGM
Thanks Rémi! I've found this by peeking at the html of a rendered screenshot of Outlook on Android in Litmus by opening the screenshot in a new tab and change .png to .html. Doesn't work for Outlook on iOS though.