CSS block formatting context

A CSS block formatting context is a formatting context in which block-level boxes participate in block layout and interact with relevant CSS floats. A box establishing a new context gives its contents an independent layout context. This is distinct from simply being a CSS block container or an ordinary block-level box.^[ref]

CSS flow-root, selected by display: flow-root, explicitly creates a block container establishing a new CSS block formatting context. An ordinary CSS inline-block box similarly combines an inline outer role with an independent flow context for its contents.^[ref]

A useful controlled layout experiment gives a parent width: 200px; height: auto; margin: 0; padding: 0; border: 0 and only one child, with float: left; width: 80px; height: 60px; margin: 0. With no other content or constraints, compare ordinary display: block with display: flow-root on the parent. The ordinary parent's content height is zero in this setup; the context-establishing parent's content height includes the sixty-pixel float. The child remains floated in both cases. This is a predicted result of the stated layout rules, rather than a browser timing measurement.^[ref][ref]

Margins do not collapse across the context boundary, but eligible margins of sibling blocks inside it can still collapse. For example, adjoining internal bottom/top margins of twenty and thirty CSS pixels can still give a thirty-pixel separation. Establishing a context is therefore not a universal switch disabling CSS margin collapsing.^[ref][ref]

Finally, CSS flow-root itself does not request clipping or a scrollbar. Treat formatting isolation, CSS containing block geometry and CSS overflow as separate properties of the layout.