all:
pre-header not hidden on mobile
should we have the metadata block in there as default?
some links missing target="_blank"

fulltemplate_650:
not fully responsive for medium and small mobile screen sizes (I think it's the social icons) - this probably doesn't matter as it's not going to be used in anger

aero:
%%xtyear%% in footer - not sure this works - should it be this instead? %%=v(@currentYear)=%%
buttons have some extra vertical padding on some mobile emails clients: https://litmus.com/pub/Q5kJxzIrG24h7k6M
button text illegible on dark mode (see litmus link above)

carnation:
not fully responsive for small mobile screen sizes [320px] (I think it's the social icons or possibly the large/wide 'make me' button)
Outlook (Windows): buttons losing full rounded corners; arksize attribute in vml probably needs increasing: https://litmus.com/pub/XOXXan80DVt4myEz

cereals:
buttons have a 1px white border around them
not responsive at all on mobile
[haven't litmus tested cos of mobile issue above]

maggi:
extra closing td/tr/table tags at bottom of HTML
'hero image left' module button no left padding on mobile
serious width issue on litmus Outlook (Windows) (width: 100%; on the header logo is doing it): https://litmus.com/pub/nwjZlVF54T7U9G3A

milkybar:
not quite fully responsive on small mobile screens (I think the big button is to blame)
widowed words in the footer on small/medium mobile screens
'hero image left' module button no left padding on mobile
width issue on litmus Outlook (Windows) (width: 100%; on the header logo is doing it): https://litmus.com/pub/X0oTb2CcsAGBqsWN

quality st.:
litmus: https://litmus.com/pub/bXLSabrjdFc6YETO
text going times new roman on Outlook (Windows)

san pell:
litmus: https://litmus.com/pub/fzkehCdVEHz8r1x7
looks great!

smarties:
large buttons stopping responsiveness on small/medium screens
litmus: https://litmus.com/pub/ptLJli1Dm1lsq22j
buttons have two background colours in Outlook (Windows) - probably a VML issue
buttons not showing in office 365 webmail clients - not sure why

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code notes:
bgcolour attributes should be style="background-color: #color;"
box-sizing: border-box /!important; has been - just worth noting this isn't supported on AOL/Yahoo/Outlook(Windows). 
empty class="" attributes - won't cause any issues, but it just feels wrong to me :)