I'd like to suggest adding support for CSS Grid Layout Module Level 2 (https://www.w3.org/TR/css-grid-2/).
It has been implemented by Firefox/Gecko and ships in Firefox 69 (https://bugzilla.mozilla.org/show_bug.cgi?id=1496502).
Chromium seems to be working on it, albeit slowly / not prioritized (https://bugs.chromium.org/p/chromium/issues/detail?id=618969).
I think this will improve mobile and responsive data layouts significantly.
Subgrid makes the grid layout specification much more useful across a wide range of use-cases. Without subgrid, authors resort to non-semantic markup, `display: contents`, and explicit sizing to get even a small fraction of what it can do. This can be the difference between spending minutes or days building a layout properly.
With support landing in Firefox 70 (Dec 3) it would be lovely to see compatibility in other browsers.