Layout web - and mobile apps
Creating layouts is often implemented in different ways on different platforms. This makes it hard to be productive. Especially the Android layout system and the iOS layout system (AutoLayout) has a serious learning curve while those skills are not reusable on the web.
Flex Box Layout system was first introduced for the web by W3C for CSS. Some frameworks for native app development implementing Flex Box are available as well. We expect it to become the standard mechanism for all platforms on the long term.
Flex Box in Android native apps
Google FlexboxLayout has worked well for one of our client projects.
Flex Box in iOS native apps
Facebook has created Yoga.
Warning: Facebook has a patents limitation on their Open Source components, always ask the owner of the software if this is acceptable. See also: this InfoQ article. Use of Facebook Open Source components that have the PATENTS limitation in its license is not permitted for products owned by Squins IT Solutions B.V.
Debugging Auto Layout applications
For some customers we have apps still build with Auto Layout.
Helpful tool to debug Auto Layout issues: wtfautolayout.com
Bootstrap 4 has Flex box support.
Google Material Design seems to have no Flex Box support. Material Design for Bootstrap 4 might be worth giving a try.