I’ve noticed that most layout is based on floats (e.g. header, nav pill etc) and all spacing is aligned via margins.
Considering that flexbox support is roughly the same as Discourse (i.e. IE10+), I wonder what are the reasons behind the decision to use one ove the other.
The reason is historical. We originally shipped with ghetto IE8 support.
Ohhhhh, I see. Any plan of refactoring to flex?
Or that’s set in stone? 
Nothing is set in stone, recently we redid the composer in flex we may get to the rest of the app at some point in time.
Not sure this is correct, I don’t think we ever shipped with IE8 support, but IE9.