Unfortunately, though flexbox is nice and a welcome addition, it's not the panacea people make it out to be having used it a fair bit in the past - the details of how it plays with the rest of the layout and the various options are complicated (see http://www.w3.org/TR/css-flexbox-1/ if you don't believe me), and sometimes the interactions are confusing.

I still think there's scope for a language that compiles to CSS but allows totally intuitive positioning, provides error messages if you make a mistake and generally isn't as infuriating as CSS is. The amount of times I've done something which seemed like it really ought to do what I want and the layout doesn't change is unbelievable. Even assuming vertical alignment is totally solved by flexbox it's still an agonisingly painful thing to work with.