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And if I were building a site myself, that's how I'd work.

Unfortunately people outside the software development team don't think in markup, and they're the ones from whom the requirements often come.

Even more technical users often base their designs on the worlds of desktop software, or type-setting, or desktop publishing, or a myriad of other domains where "I want these two boxes to always be the same size and aligned to the bottom of this third box, and I'd like the font size to fit across the resultant size" is a perfectly reasonable request.

We can't keep going saying "Well the web doesn't work that way, you're going to have to rethink how you think about sizing" all the time, eventually the web has fix itself so that sizing and aligning fits better with how a lot of other domains work.

Flexbox is a step in the right direction but it's not a complete solution. (And we're still supporting IE9, so we can't even use that yet, thanks Vista!).