This spec introduces a way to pass CSS values into linked resources, such as SVG images, so that they can be used as CSS custom property values in the destination resource. This allows easy reuse of "templated" SVG images, which can be adapted to a site’s theme color, etc. easily, without having to modify the source SVG.
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-link-params” in the title, like this:
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SVG is stylable with CSS,
and when used inline in HTML,
this capability can be very useful.
For example, an SVG icon can take on a different color based on whether the user is hovering it or not,
just by applying a :hover rule to it that changes the fill property.
When the SVG is referenced in a way that doesn’t allow selectors or CSS inheritance from the outer page to apply to it
(such as embedding it via img or iframe in HTML),
though, this functionality is lost.
The only way to change the display of such "external" SVG images
is to produce several of them,
and change which image you’re referencing.
This incurs delay on the page as a new resource is downloaded,
and disallows dynamic effects like CSS Transitions.
CSS link parameters are a way to set CSS custom properties on an "external" resource,
either by a CSS property
or thru a special fragment scheme on the URL.
This gives a limited, but powerful, subset of the customizability that "inline" SVG images have
to "external" SVG images.
via the link-parameters property,
which applies to the resource itself
(if the element represents an external resource),
and to all external resources used in CSS properties on the element
via a special syntax in the fragment portion of the URL of an external resource
If specified in multiple of these ways,
all of the link parameters are combined.
If the same key appears in multiple inputs,
the latest source in the above list wins
(that is, URL fragment beats link-parameters,
and url("..." param()) beats URL fragment).
The influence of link parameters on the linked resource
is defined in the next section.
The link-parameters property is one way to set link parameters on the element itself
(if it is an element representing an external resource,
such as an HTML img or iframe),
and on all external CSS resources specified on the element
(such as background images, etc).
Its values are:
A list of one or more link parameters.
If two link parameters with the same name are specified
with the same <custom-property-name>,
the last one wins.
A special "fragment identifier" can be used in the fragment of a URL
used to reference an external resource.
Several examples of existing "fragment identifiers" for SVG documents can be found in the SVG 1.1 specification.
The syntax of an SVG parameter fragment identifier is:
For example, to set the "--text-color" custom property of an SVG image to blue,
one can reference the image with a url like “http://example.com/image.svg#param(--text-color%20blue)”.
If passing multiple parameters to an image,
additional param() functions must be appended to the URL.
If multiple param() functions specify the same <custom-property-name>,
the custom property is set to the value of the last one.
For example, if the image from the previous example also used a "--bg-color" custom property,
it could be referenced with a url like “http://example.com/image.svg#param(--text-color%20blue)param(--bg-color%20white)”.
Note: Spaces, and some other characters that might be valid in CSS syntax,
are not technically valid in URLs.
In some contexts,
you might need to escape those characters to form a valid URL.
In most cases, though,
such as HTML’s a element or CSS’s url() function,
spaces are accepted and do not need to be escaped.
When referencing an external resource via CSS,
the param() function can be used in the url() function.
But a common use-case is passing in values of the page’s own custom properties;
for example, a page might use a --primary-colorcustom property,
and want to make an SVG image match.
There’s no way, however, to integrate the value of a custom property in CSS into the URL passed to the url() function.
For example,
if the site is using a --primary-color custom property to theme its elements with,
and wanted an SVG background using a --color custom property to reflect it,
it could write:
When an external resource link has one or more link parameters specified,
if the linked resource understands CSS
(such as an SVG or HTML document),
then the initial value of custom properties
with names equal to the keys of the link parameters map
is set to the corresponding values of the map.
If an @property rule is specified for one of the custom property names
in the link parameters,
the link parameter value is used for the initial value,
rather than the @property-specified initial value.
If the linked resource does not understand CSS
(such as PNG images),
then link parameters have no effect.
Define a way for the linked resource to specify what link parameters they allow.
For cross-origin iframes/etc,
this will default to nothing;
for same-origin (or cross-origin "SVG as image"),
it defaults to "everything".
If not allowed, the link parameter is ignored.
For example, if an SVG image wanted to expose a --color parameter,
it could use it like:
It’s usually a good idea to make your SVG image usable even if no parameters are given,
by providing "default values" for each of the custom properties.
There are several ways to do this.
If the custom property is going to be used a lot,
such that providing a fallback for each individual var() is troublesome,
store the custom property in a different name while invoking the default,
like:
:root {--color2:var(--color, blue);}
In this example, if --color is provided via an SVG parameter, --color2 will receive its value.
If not, it will recieve the default blue value.
In either case, --color2 can be used in the SVG image’s stylesheet unconditionally,
secure in the knowledge that it will always have a value.
In a future level of the Custom Properties specification [CSS-VARIABLES],
some "parent’s value" functionality will be available to make the previous suggestion more usable:
:root {--color:var(parent --color, blue);}
(This is an example syntax, and is not yet final.)
By invoking the value of the --color property on the parent
(which, on the root element, refers to the initial value),
an author can avoid self-reference loops while retaining the same custom property name.
Conformance
Document conventions
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 a way for the linked resource to specify what link parameters they allow.
For cross-origin iframes/etc,
this will default to nothing;
for same-origin (or cross-origin "SVG as image"),
it defaults to "everything".
If not allowed, the link parameter is ignored. ↵