Your content will flow through various containers (called regions) which you specify.
The CSS regions module allows content to flow across multiple areas called regions. The regions are not necessarily contiguous in the document order. The CSS regions module provides an advanced content flow mechanism, which can be combined with positioning schemes as defined by other CSS modules such as the Multi-Column Module [CSS3COL] or the Grid Layout Module [CSS3-GRID-LAYOUT] to position the regions where content flows.
Imagine a magazine layout where you could have multiple columns in your article or where the cut of a paragraph continues to the next page.
With all these CSS developments going on, one could really build a web app based on HTML and CSS as its front-end.
We’re really in the 21st century.
Also, checkout Adobe & HTML’s page on CSS Regions.