Note that this answer uses the term "box" in lieu of "element", as CSS makes a distinction between elements and boxes. For the purposes of this answer, an HTML element is represented by a single box in CSS layout. In reality an element may generate any number of boxes (or none at all, as in display: none), but that's outside the scope of this question.
Can a HTML element be both a block level element and form a block formatting context?
Yes. The criteria in which a block box (i.e. a block-level block container box) may establish a BFC are stated in section 9.4.1 of CSS2.1, namely:
- floats,
- absolutely positioned elements, and
- "block boxes with 'overflow' other than 'visible' (except when that value has been propagated to the viewport)" (as directly quoted from the spec)
Does being a block level element imply that it forms a block formatting context, or conversely, does forming a block formatting context imply that it must be a block level element?
Neither:
- The above answer implies that not all block boxes establish block formatting contexts. A block box with the CSS properties
display: block; overflow: visible; float: none; position: static (or position: relative) does not establish a BFC.
- Conversely, an inline-block is an example of a box that establishes a BFC, but is itself inline-level, not block-level.
In a similar vein, how does this translate to inline elements and elements that form an inline formatting context?
An inline box is an inline-level box whose contents participate directly in its parent's inline formatting context (see section 9.4.2). Notably, the only boxes that can establish inline formatting contexts are block container boxes.
The difference between an inline box and an inline-block is that an inline-block's contents participate in the BFC that it establishes, not in the parent's IFC. Instead, only the inline-block itself participates in its parent's IFC.
