Other new technologies impacting web application development are two new CSS layout methodologies. The CSS3 committee has been working on several fronts, including page layout. 

In the distant past, we used nested HTML tables for page layout. That is a bad memory that we don't have to revisit. More recently, we've been using a box model using DIVs, and even at times using absolute or relative placement techniques. All these techniques have been suboptimal in several ways, some more than others.

One popular layout technique is to divide the horizontal space into columns and assign a certain number of columns to each thing on the page. With some frameworks, we can even have nested DIVs, each with their own set of columns. Bootstrap 3, and other modern frameworks, used that layout technique. 

The two new CSS layout methodologies, Flexbox (https://en.wikipedia.org/wiki/CSS_flex-box_layout) and CSS Grids (https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_Layout), are a significant improvement over all previous methodologies. We are mentioning these technologies because they're both worthy of attention. Both are somewhat early in their adoption curve—they've been standardized by committees and adopted in the latest browsers, but of course there are a lot of old browsers in the field.

With Bootstrap 4, the Bootstrap team chose to go with Flexbox. Therefore, under the hood are Flexbox CSS constructs.