In this guide, we will explore the basics of how Block and Inline elements behave when they are part of the normal flow.
Normal Flow is defined in the CSS 2.1 specification, which explains that any boxes in normal flow will be part of a formatting context. They can be either block or inline, but not both at once. We describe block-level boxes as participating in a block formatting context, and inline-level boxes as participating in an inline formatting context.
The behavior of elements which have a block or inline formatting context is also defined in this specification. For elements with a block formatting context, the spec says:
"In a block formatting context, boxes are laid out one after the other, vertically, beginning at the top of a containing block. The vertical distance between two sibling boxes is determined by the 'margin' properties. Vertical margins between adjacent block-level boxes in a block formatting context collapse.
In a block formatting context, each box's left outer edge touches the left edge of the containing block (for right-to-left formatting, right edges touch)." - 9.4.1
For elements with an inline formatting context:
"In an inline formatting context, boxes are laid out horizontally, one after the other, beginning at the top of a containing block. Horizontal margins, borders, and padding are respected between these boxes. The boxes may be aligned vertically in different ways: their bottoms or tops may be aligned, or the baselines of text within them may be aligned. The rectangular area that contains the boxes that form a line is called a line box." - 9.4.2
Note that the CSS 2.1 specification describes documents as being in a horizontal, top to bottom writing mode. For example, by describing vertical distance between block boxes. The behavior on block and inline elements is the same when working in a vertical writing mode, and we will explore this in a future guide on Flow Layout and Writing Modes.
Elements participating in a block formatting context
Block elements in a horizontal writing mode such as English, layout vertically, one below the other.
In a vertical writing mode then would lay out horizontally.
In this guide, we will be working in English and therefore a horizontal writing mode. However, everything described should work in the same way if your document is in a vertical writing mode.
As defined in the specification, the margins between two block boxes are what creates separation between the elements. We see this with a very simple layout of two paragraphs, to which I have added a border. The default browser stylesheet adds spacing between the paragraphs by way of adding a margin to the top and bottom.
If we set margins on the paragraph element to 0 then the borders will touch.
By default block elements will consume all of the space in the inline direction, so our paragraphs spread out and get as big as they can inside their containing block. If we give them a width, they will continue to lay out one below the other - even if there would be space for them to be side by side. Each will start against the start edge of the containing block, so the place at which sentences would begin in that writing mode.
The display property and flow layout
In addition to the rules existing in CSS2.1, new levels of CSS further describe the behavior of block and inline boxes. The display property defines how a box and any boxes inside it behave. In the CSS Display Model Level 3, we can learn more about how the display property changes the behavior of boxes and the boxes they generate.
The display type of an element defines the outer display type; this dictates how the box displays alongside other elements in the same formatting context. It also defines the inner display type, which dictates how boxes inside this element behave. We can see this very clearly when considering a flex layout. In the example below I have a <div>, which I have given display: flex. The flex container behaves like a block element: it displays on a new line and takes up all of the space it can in the inline direction. This is the outer display type of block.
The flex items however are participating in a flex formatting context, because their parent is the element with display: flex, which has an inner display type of flex, establishing the flex formatting context for the direct children.
Therefore you can think of every box in CSS working in this way. The box itself has an outer display type, so it knows how to behave alongside other boxes. It then has an inner display type which changes the way its children behave. Those children then have an outer and inner display type too. The flex items in the previous example become flex level boxes, so their outer display type is dictated by way of them being part of the flex formatting context. They have an inner display type of flow however, meaning that their children participate in normal flow. Items nested inside our flex item lay themselves out as block and inline elements unless something changes their display type.
This concept of the outer and inner display type is important as this tells us that a container using a layout method such as Flexbox (display: flex) and Grid Layout (display: grid) is still participating in block and inline layout, due to the outer display type of those methods being block.
