[c-smile]

Correct CSS grid layout calculation is about solving system of equations and constraints. In simple cases, when there are no spanned cells (like in flexbox), it can be done relatively trivially.

Otherwise solving that system is far from being trivial, you will need simplex solver or the like, for example https://constraints.cs.washington.edu/solvers/cassowary-toch...

[nicoburns]

You definitely don't need a general constraint solver for CSS Grid. The algorithm (including for cases where there are spanned cells) is well defined in the spec [1], and can be translated directly into code.

1: https://www.w3.org/TR/css-grid-1/#algo-content