Should CSS be a constraint system instead?

IMO what CSS layout really needs is:

1. A "proportion of available space" unit. That is, something like the fr unit in CSS grid except applicable to the width/height properties so it can be used in all layout modes and without an implying content-based minimum size (like fr does).

2. A new "display: stack" (name provisional) layout mode that simply stacks boxes one after the other like "display: block" but that works in both axes, and doesn't have quirks of block layout (margin-collapsing, floats, inline-block splitting, etc).

When combining 1 and 2 you'd have a much more intutive layout system that would still give you most of the power of Flexbox and could be implemented with much better performance.

(and you'd still be able to use the existing layout modes if/when you needed to extra power)

Couple those together and you get poor performance. Or more specifically, unpredictable performance with lots of performance pitfalls that are hard to debug. Apple's AutoLayout is the real-world case study for this.

My guess is provides the power to do layouts that are difficult to do in CSS and also more computationally expensive, so it's not that constraints are slower, but that doing more complex layout requires more computation.

Edit: https://microsoft.github.io/apple-ux-guide/Layout.html (FWIW, by Microsoft?) seems to confirm this. Performance for normal layouts is normal, doing complex things with lots of chained, dependent constraints that modify many things is slow.

So, I don't think this is a good argument against constraint systems. People can do crazy things with more power. In fact, they already do crazy things because they can use Javascript which is ultimately powerful. Adding one more powerful system isn't going to change things. Companies do have limits for loading/rendering times for websites they publish, as loose as they are.

These are also the kind of cases where CSS layout ends up being slow. But a constraint solver based layout gives you more power to shoot yourself in the foot with.

> but that doing more complex layout requires more computation.

It's exactly this. The question is whether that makes for an ergonomic system to use for the developer. My assertion is that if there is no feedback when you create a slow layout, then it is not actually an easy system to use, and you're better off with something more constrained that guides you into the pit of success.