We use small artboards while the devices we design for are huge. For example:

• The iPhone XR has a Retina display that has twice as many pixels per inch (ppi) as a regular screen, meaning the artboard is rendered at twice as many pixels.
• On the iPhone X, the artboard is rendered...

While the font sizes can vary and move away from the 8 multiplication, the line height will not. 

For example, the Paragraph font size can be 15 pixels, but the line height should be a multiplication of 8, so 24px is an acceptable value. 

The standard Bootstrap grid is a 12 column layout, with a 15px margin on each side of the column, resulting in a 30px gutter between columns plus 15px to the left and right of the grid. 

The entirety of the grid is called Container. A container can be either fluid or responsive in size. A f...

• It scales perfectly in all the different screen displays (including android’s @0.75 and @1.5).
• Apple and Google advise so.
• It's a good basic unit to work with. 4 and 8 are easily multiplied, are flexible and consistent. 

Rows should be 8px tall. 

Maintaining a vertical rhythm when designing for mobile (web or apps) is vital since there isn’t much horizontal rhythm to work with.

A typographic system that is designed based on the 8pt grid will sit wonderfully on an 8pt vertical grid.