The code shown on that page actually doesn't work, though the code in the github repo does.
And this exposes the problem with the current state of the art: yes, you can probably get flexbox to do what you want [@] if you know how. But if you don't know how, trying to figure it out is still very hard. You should be able to write:
<center>stuff</center>
or
<vertical-center>stuff</vertical-center>
There is really no reason that should not Just Work. (The <center> tag has worked for 20 years, notwithstanding that it has been deprecated for most of that time.)
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[@] What if I want to put something in the lower left or upper right hand corner?
And this exposes the problem with the current state of the art: yes, you can probably get flexbox to do what you want [@] if you know how. But if you don't know how, trying to figure it out is still very hard. You should be able to write:
or There is really no reason that should not Just Work. (The <center> tag has worked for 20 years, notwithstanding that it has been deprecated for most of that time.)---
[@] What if I want to put something in the lower left or upper right hand corner?