> so the usage of margin-top should always be avoided

Not really if you consider sibling layouts, e.g.: https://matthewjamestaylor.com/css-margin-top-vs-bottom

> owl selector: performance implications - it's fairly bad from a performance POV to use it.

Arguing about "CSS performance" is futile when the real cause of bloatedness is mostly JavaScript these days (also: http://alistapart.com/article/axiomatic-css-and-lobotomized-...).

The owl-selector can actually be quite elegant, but becomes a burden if you have lots of side-by-side blocks (which would always need to set their margin-top to 0).