From @chriscoyier
"One of the Boss Battles of #CSS is Almost Won! Transitioning to Auto"
https://frontendmasters.com/blog/one-of-the-boss-battles-of-css-is-almost-won-transitioning-to-auto/
"The experimental CSS function calc-size(auto) can be used so that transitions and animations can go from zero (0) to this value. But that is unlikely to be the final syntax! ⚠️ So be forewarned."