Has anyone come across a working template for Asymmetrical Columns?

Say Column one is 25% width (as disctated by max-width also) and Column Two is 75% (also containing a max width), all of which is conducive to stacking 100% rows for Mobile view.
E.G.

  • (A) 25% - (B) 75% -

TO:

  • (A) 100% -
  • (B) 100% -

The crux of all of this is for this layout to work in Gmail App and Gmail Desktop?
Thanks,

  • Jonny