Logical properties for floating and positioning

The table below details the properties and values discussed in this guide along with their physical mappings. They assume a horizontal writing-mode, with a left-to-right direction.

Logical property or value	Physical property or value
float: inline-start	float: left
float: inline-end	float: right
clear: inline-start	clear: left
clear: inline-end	clear: right
inset-inline-start	left
inset-inline-end	right
inset-block-start	top
inset-block-end	bottom
text-align: start	text-align: left
text-align: end	text-align: right

In addition to these mapped properties there are some additional shorthand properties made possible by being able to address block and inline dimensions. These have no mapping to physical properties, aside from the inset property.

Logical property	Purpose
inset-inline	Sets both of the above inset values for the inline dimension simultaneously.
inset-block	Sets both of the above inset values for the block dimension simultaneously.
inset	Sets all four inset values simultaneously with physical mapping of multi-value.

The physical values used with the float and clear properties are left, right and both. The Logical Properties specification defines the values inline-start and inline-end as mappings for left and right.

In the example below I have two boxes — the first has the box floated with float: left, the second with float: inline-start. If you change the writing-mode to vertical-rl or the direction to rtl you will see that the left-floated box always sticks to the left, whereas the inline-start-floated item follows the direction and writing-mode.

Positioning generally allows us to position an element in a manner relative to its containing block — we essentially inset the item relative to where it would fall based on normal flow. To do this we have historically used the physical properties top, right, bottom and left.

These properties take a length or a percentage as a value, and relate to the user's screen dimensions.

New properties have been created in the Logical Properties specification for when you want the positioning to relate to the flow of text in your writing mode. These are as follows: inset-block-start, inset-block-end, inset-inline-start and inset-inline-end.

In the below example I have used the inset-block-start and inset-inline-end properties to position the blue box using absolute positioning inside the area with the grey dotted border, which has position: relative. Change the writing-mode property to vertical-rl, or add direction: rtl, and see how the flow relative box stays with the text direction.

As with other properties in the specification we have some new shorthand properties, which give the ability to set two or four values at once.

• inset — sets all four sides together with physical mapping.
• inset-inline — sets both logical inline insets.
• inset-block — sets both logical block insets.

The text-align property has logical values that relate to text direction — rather than using left and right we can use start and end. In the below example I have set text-align: right in the first block and text-align: end in the second.

If you change the value of direction to rtl you will see that the alignment stays to the right for the first block, but goes to the logical end on the left in the second.

This works more consistently when using box alignment that uses start and end rather than physical directions for alignment.