Let's implement a FlowLayout/HorizontalLayout in OOjs UI, that would arrange its contents in a single line (using display: inline-block for its items), with small margin inbetween them. It would be more convenient than using CSS for it all over the place.

Thoughts:

• Should it be a Layout or a Widget? A Layout would be "more correct", maybe, but it couldn't be directly used inside a FieldLayout, for example (you'd need to create a custom widget to "wrap" it; which might be a good or bad thing, depending on how you look at it).
• Te name "FlowLayout" was inspired by an obscure GUI toolkit I'm fond of :P, which uses "stacks" and "flows" to lay out window content (http://shoesrb.com/manual/Slots.html), and we already have a StackLayout. "HorizontalLayout" might be more obvious.