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Inner And Outer Values Of `display`

Rachel Andrew explains what really happens when we change the value of `display` on an element in light of the two-value syntax.

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Recently, Stefan Judis posted on Mastodon:

Hand to the heart: is anyone using multi-keyword display values? Because I haven’t seem them anywhere but the MDN docs. 🫣

.element {
	/* multi-keyword syntax */
	display: block flex;
	display: block flow;
	display: block flow-root;
	display: block grid;
	display: inline flex;
	display: inline flow;
	display: inline flow-root;
	display: inline grid;

I hadn’t seen this two-value syntax until I saw Stefan’s post, and I wanted to know more. Fortunately, Rachel Andrew replied to Stefan’s post, sharing that they had written about this topic several years ago.

They explain in “Digging Into The Display Property: The Two Values Of Display”:

In Level 3 of the Display specification, the value of display is defined as two keywords. These keywords define the outer value of display, which will be inline or block and therefore define how the element behaves in the layout alongside other elements. They also define the inner value of the element — or how the direct children of that element behave.

This means that when you say display: grid, what you are really saying is display: block grid. You are asking for a block level grid container. An element that will have all of the block attributes — you can give it height and width, margin and padding, and it will stretch to fill the container. The children of that container, however, have been given the inner value of grid so they become grid items. How those grid items behave is defined in the CSS Grid Specification: the display spec gives us a way to tell the browser that this is the layout method we want to use.

As simply put by Rachel:

When you define layout on a box in CSS, you are defining what happens to this box in terms of how it behaves in relation to all of the other boxes in the layout. You are also defining how the children of that box behave.

Browser Support

As per Can I Use and Web Platform Dashboard, browsers do support the two-value syntax for the display property since 21 July 2023.

More on the display property

In addition to everything mentioned above, Rachel’s article also explains block and inline layout, flow-root and inline-block. It’s an essential read and I’m glad I read it.

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