Hi Sergey,
Thanks for that, you are right, that fixes another issue I had with this layout. However my question was about getting the gutter between .intro and .sidebar under the .main-content divs to line up with the four .tile divs under the .featured-tiles div.
I’d basically be happy if there was a way to solve this in flexbox without resorting to fixed widths for the containers or without being obliged to add special wrapper divs… But I’m not actually sure it’s possible in which case it’s putting us back into the realm of non semantic markup, the very thing we are trying to avoid.
You got any ideas? :-o
I can only imagine that we have to correct the percentages so that things line up. This is an easy calculation, but starts to get messy when you want to add in a fixed with gutter based on px etc. Not to mention dealing with responsive design.
Perhaps I should look at how this achieved in burbon neat or bulma…
In any case, thanks for your answer! 
Would love to hear any thoughts, pointers from the community,
Cheers,
Brian.