Nice tutorial on understanding flexbox for better responsive designs.
David Bainbridge's profile photoRamsez Stamper's profile photo
And my usual response: flexbox is not fully supported by all browsers. Especially the bit older versions of IE which still make up a sizable percentage of the market.

Use inline block to display things responsively and line heights to vertically center text or maybe the position hack.

It doesn't matter if your code is all pretty and on the frontline of new things if a portion of users are having a shitty experience.
+Ramsez Stamper I agree you want to get the best support across all browsers, flexbox is fairly widely supported now though, see here: The only browser with some limitations is IE and it's only versions 9 and older that don't support it at all accounting for less than 0.5% of users. I don't think coding standards should be dictated by IE9 :)
Depends on your market that percentage would be much higher
True, but that goes both ways. My personal stats show a lot less. Depends on your audience but I don't think it will be significant for the majority.
3 different sources disagree with that statement:

It varies, but IE versions below 10 in some cases add up to more than 5%.

It may not affect you, but in many countries IE is still very measurable. 1 in 20 users having a problem, is a big problem.

Why not build it with code that simply works for all of them? What's the argument against that?

My argument would be that minority, out-of-date browsers shouldn't hold back using newer and better technologies, even Google Analytics doesn't support ie9, if they did they would probably have to limit features for everyone else. Don't get me wrong, if I was working on a site where accessibility was paramount, say for a public sector site, then fine tick all the boxes, but in most other cases I'd sooner notify the user to upgrade their browser - as of this month even Microsoft have dropped support for IE9.
I'm not saying stay away from new tech, but google's code not working in IE9 is a choice google made. And they are big enough to where people just suck it up and keep using them. Most companies do not have that kinda clout.

I'm mostly against using code that can be done differently and still work for everything new AND old. It's not like display:inline-block is going away any time soon, or line-height, or positioning. 
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