main axis – The main axis of a flex container is the primary axis along which flex items are laid out.Items will be laid out following either the main axis (from main-start to main-end) or the cross axis (from cross-start to cross-end). Please have a look at this figure from the specification, explaining the main idea behind the flex layout.

If “regular” layout is based on both block and inline flow directions, the flex layout is based on “flex-flow directions”. Some of them are meant to be set on the container (parent element, known as “flex container”) whereas the others are meant to be set on the children (said “flex items”). Since flexbox is a whole module and not a single property, it involves a lot of things including its whole set of properties. Note: Flexbox layout is most appropriate to the components of an application, and small-scale layouts, while the Grid layout is intended for larger scale layouts. While those work well for pages, they lack flexibility (no pun intended) to support large or complex applications (especially when it comes to orientation changing, resizing, stretching, shrinking, etc.).

Most importantly, the flexbox layout is direction-agnostic as opposed to the regular layouts (block which is vertically-based and inline which is horizontally-based). A flex container expands items to fill available free space or shrinks them to prevent overflow. The main idea behind the flex layout is to give the container the ability to alter its items’ width/height (and order) to best fill the available space (mostly to accommodate to all kind of display devices and screen sizes). The Flexbox Layout (Flexible Box) module ( a W3C Candidate Recommendation as of October 2017) aims at providing a more efficient way to lay out, align and distribute space among items in a container, even when their size is unknown and/or dynamic (thus the word “flex”).

Css display flex vertical align