I've been using the experimental "display: masonry" CSS for my gallery and a few other areas of my website for over a year (with a fallback to standard grid columns). However, I noticed in the last few months that my browser was no longer honoring it, and finally researched what happened.

It turns out it's been renamed to "display: grid-lanes" in the CSS Grid Level 3 specification. Otherwise, everything else remains the same; continue using grid-template-columns and grid-template-rows as you did with the the masonry display.

Safari has it in preview already, while Firefox and Chrome-based browsers have it behind a flag. (On Chrome-based browsers, visit chrome://flags, and enable the "css-grid-lanes-layout" flag.)