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.
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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 thruout 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.
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.
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.
The second argument, 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,
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.
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.
Requirements for Responsible Implementation of CSS
The following sections define several conformance requirements
for implementing CSS responsibly,
in a way that promotes interoperability in the present and future.
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 property 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,
implementers should release an unprefixed implementation
of any CR-level feature they can demonstrate
to be correctly implemented according to spec,
and should avoid exposing a prefixed variant of that feature.
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. ↵