This specification defines the concept of environment variables and the env() function, which work similarly to custom properties and the var() function, but are defined globally for a document. These can be defined either by the user agent, providing values that can be used on the page based on information the UA has special access to, or provided by the author for "global" variables that are guaranteed to be the same no matter where in the document they’re used.
CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
Status of this document
This is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don’t cite this document other than as work in progress.
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including the spec code “css-env” in the title, like this:
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Alternately, feedback can be sent to the (archived) public mailing list [email protected].
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.)
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.
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.
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.
When the viewport is split into two side-by-side segments, the viewport segment on
the left would have indices (0, 0). It’s width would be represented as env(viewport-segment-width 0 0, 300px).
The viewport segment on the right would have indices (1, 0).
Similarly, for a viewport split into two vertical segments, the viewport segment
on the top would have indices (0, 0) and the one on the bottom (0, 1).
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:
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.
Should be able to replace any subset of MQ syntax, for example.
Should be able to replace selectors, maybe?
Should it work on a rule level,
so you can insert arbitrary stuff into a rule,
like reusing a block of declarations?
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.
To substitute an env() in a property or descriptor:
If the name provided by the first argument of the env() function
is a recognized environment variable name, the number of supplied integers
matches the number of dimensions of the environment variable referenced
by that name, and values of the indices correspond to a known sub-value,
replace the env() function by the value of the named environment variable.
Otherwise, if the env() function has a fallback value as its second argument,
replace the env() function by the fallback value.
If there are any env() references in the fallback, substitute them as well.
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.
Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with class="example",
like this:
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from the
normative text with class="note", like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <strong class="advisement">, like
this: UAs MUST provide an accessible alternative.
Tests
Tests relating to the content of this specification
may be documented in “Tests” blocks like this one.
Any such block is non-normative.
Conformance classes
Conformance to this specification
is defined for three conformance classes:
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
Partial implementations
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must treat as invalid (and ignore
as appropriate) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents must not selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.
Implementations of Unstable and Proprietary Features
Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.
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... ↵
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. ↵
If env() substitution happens during parsing,
then this is unnecessary. ↵
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