[rachelandrew]

That would depend entirely on the browsers you need to support. For example I have a product aimed at web designers. Our support in terms of our marketing site for the up to date flexbox spec is about 98%.

Our product is used by their clients. Some of those people are using IE8 and 9. So the approach we take for our product marketing site and the product itself is different.

Flexbox however isn't really designed for full page layout. The UI items it is really great at are often much easier to deal with in terms of legacy support than you entire layout is. You can often use flexbox to enhance a design even if you can't go all in. I would really encourage you to have a look at this video https://www.youtube.com/watch?v=_98SE8WUvLk Zoe works for booking.com and demonstrates how they use flexbox AND ensure legacy support.