What's the difference between a block-level box and a principal block-level box?

Question

To cite the spec:

Block-level boxes are boxes that participate in a block formatting context. Each block-level element generates a principal block-l

Answer 1

principal block is essentially what your 'markup' represents, for mechanical purposes, some 'principal' elements needs other elements to be rendered correctly, those are some-kind of 'artifacts' needed by a browser to display extra ( a scroll bar from a textarea for example )