[danShumway]

So, I would push back against this as a design pattern, but I do want to re-emphasize, if you really want to absolute-position all of your divs, and if you want to serve even completely separate HTML for different devices -- you can.

That is a thing that CSS allows. Flexbox, responsive design -- it's all optional. There is going to be a sense when you use CSS that way that you are swimming against the grain, and that's on purpose because it's important that the instinctive default way that new designers think about the web is in those responsive terms. But even if the web pushes you in that direction a bit, it's not a requirement -- you can still design interfaces the way you're talking about, and you can heavily use breakpoints and you can have different layouts depending on the situation.

One thing that I see websites do fairly often (which I don't think is great practice but I'm throwing it out as a common practice and I understand why people do it) is literally have two versions of their menus for mobile and desktop in the same interface in the same DOM tree and just throw a "display: none" on one of them depending on the width of the screen.

So it's not even necessarily that you have to serve different HTML, you can have different versions of components even on a completely static website with no JS that swap out seamlessly depending on the user's device size.

[taeric]

I don't want to absolutely position all divs. But if I have one that I want to be always to the left taking up a knowable amount of space, I don't see the benefit in trying to make flow based choices so that that works out.

And fair that flexbox and such are probably fine for a lot of that, nowadays. Amusingly closer to the layout managers of early Java development days. Most of my learning was pre that. Which is why I do stress that I agree that CSS is perfectly workable.

The only real problem I see, is that sometimes the different layouts do require either slightly different markup for parenting/flex choices to work out in different layouts, or they require hilariously non-obvious markup that will work with both. Amusingly, XSLT days worked well in that aim, as you could keep the base markup only the content, but then use a stylesheet to add/remove things as necessary for display. Alas, that went nowhere.