CSS Environment Variables Module Level 1

1. Introduction

The [css-variables-1] specification defined the concept of "cascading variables", author-defined variables created from the value of custom properties, capable of being substituted into arbitrary other properties via the var() function.

This specification defines a related, but simpler, concept of environment variables. Unlike "cascading variables", which can change throughout the page as their corresponding custom property takes on different values, an environment variable is "global" to a particular document—​its value is the same everywhere. The env() function can then be used to substitute the value into arbitrary locations, similar to the var() function.

These "global" variables have both benefits and downsides versus cascading variables:

• Many variables aren’t meant to change over the course of a page; they set up themes, or are helpers for particular numerical values. Using environment variables instead of custom properties to define these communicates the proper intent, which is good both for the author of the document (particularly when multiple people are collaborating on a single document), and for the user agent, as it can store these variables in a more optimal way.

• Because environment variables don’t depend on the value of anything drawn from a particular element, they can be used in places where there is no obvious element to draw from, such as in @media rules, where the var() function would not be valid.

• Information from the user agent itself, such as the margin of the viewport to avoid laying out in by default (for example, to avoid overlapping a "notch" in the screen), can be retrieved via env(), whereas the element-specific nature of var() was not an appropriate place to pipe that information in.

Most environment variables will have a single value at a time. Some, however, are "indexed", representing multiple values at once, such as the sizes and positions of several distinct panes of content in the viewport-segment-* variables. To refer to these indexed variables, one or more integers must be provided alongside the variable name, like viewport-segment-width 1 0, to select a single value from the list or grid of possibilities, similar to selecting one element from a list in a traditional programming language with a syntax like values[0].

2. Environment Variables

A CSS environment variable is a name associated with a <declaration-value> (a sequence of zero more CSS tokens, with almost no restrictions on what tokens can exist), similar to a custom property. Environment variables can be defined by the user agent, or by the user. (In the latter case, the names are <custom-property-name>s, and start with `--` per standard for custom identifiers.)

Is the set of UA-defined environment variables visible to script? If so, define an API on Document to expose them.

Define how authors can add environment variables, preferably both via JS and via CSS. Note that mixing CSS rules and JS-defined stuff can easily get messy, as demonstrated by CSSFontFaceRule vs FontFace...

The following UA-defined environment variables are officially defined and must be supported. Additional UA-defined environment variables *must not* be supported unless/until they are added to this list.

2.1. Safe area inset variables

The safe area insets are four environment variables that define a rectangle by its top, right, bottom, and left insets from the edge of the viewport. For rectangular displays, these must all be zero, but for nonrectangular displays they must form a rectangle, chosen by the user agent, such that all content inside the rectangle is visible, and such that reducing any of the insets would cause some content inside of the rectangle to be invisible due to the nonrectangular nature of the display. This allows authors to limit the layout of essential content to the space inside of the safe area rectangle.

2.2. Viewport segment variables

The viewport segments are environment variables that define the position and dimensions of a logically separate region of the viewport. Viewport segments are created when the viewport is split by one or more hardware features (such as a fold or a hinge between separate displays) that act as a divider; segments are the regions of the viewport that can be treated as logically distinct by the author.

The viewport segment environment variables have two dimensions, which represent the x and y position, respectively, in the two dimensional grid created by the hardware features separating the segments. Segments along the left edge have x position 0, those in the next column to the right have x position 1, etc. Similarly, segments along the top edge have y position 0, etc.

Note: In certain hardware configurations, the separator itself may occupy logical space within the viewport. The dimensions of the separator can be computed by calculating the area between the position of the viewport segments.

These variables are only defined when there are at least two such segments. Viewport units should be used instead when there is no hardware feature splitting the viewport, otherwise content will not display as intended when viewed on a device with multiple segments.

3. Using Environment Variables: the env() notation

In order to substitute the value of an environment variable into a CSS context, use the env() function:

env() = env( <custom-ident> <integer [0,∞]>*, <declaration-value>? )

The env() function can be used in place of any part of a value in any property on any element, or any part of a value in any descriptor on any at-rule, and in several other places where CSS values are allowed.

The first argument to env() provides the name of an environment variable to be substituted. Following the first argument are integers that represent indices into the dimensions of the environment variable, if the provided name represents an array-like environment variable. The argument after the comma, if provided, is a fallback value, which is used as the substitution value when the referenced environment variable does not exist.

Note: The syntax of the fallback, like that of custom properties, allows commas. For example, env(foo, red, blue) defines a fallback of red, blue; that is, anything between the first comma and the end of the function is considered a fallback value.

If a property contains one or more env() functions, and those functions are syntactically valid, the entire property’s grammar must be assumed to be valid at parse time. It is only syntax-checked at computed-time, after env() functions have been substituted.

If a descriptor contains one or more env() functions, and those functions are syntactically valid, the entire declaration’s grammar must be assumed to be valid at parse time. It is only syntax-checked after env() functions have been substituted.

Define when substitution happens. It has to be before var() substitution. Alternately, should env() substitution happen at parse time, so unknown variable names cause it to fail syntax checking? There’s no particular reason to have it happen at computed-value time, like var() does—​that was to ensure that custom properties could inherit their value down before they were picked up by a var().

When I figure out where else env() can go, define how/when it substitutes.

3.1. Environment Variables in Shorthand Properties

If env() substitution happens during parsing, then this is unnecessary.

The env() function causes the same difficulties with shorthand properties as the var() function does. When an env() is used in a shorthand property, then, it has the same effects as defined in CSS Variables 1 § 3.2 Variables in Shorthand Properties.