SVG Parameters

Unofficial Proposal Draft,

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This introduces "parameters" to SVG, which are a method of setting CSS custom properties in an external SVG document via the fragment on the referencing URL. 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, in speech, etc.

Status of this document

1. Introduction

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 appling 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.

SVG Parameters are a way to set CSS custom properties on an "external" SVG image, by passing them through 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.

2. Setting an SVG Parameter

To "set" an SVG Parameter, a special "fragment identifier" must be used in the fragment of the URL used to reference. 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:

param( <custom-property-name> <declaration-value> )

(using the CSS value definition syntax).

For example, to set the "--text-color" custom property of an SVG image to blue, one can reference the image with a url like “”.

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 “”.

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 a valid param() function is encountered in the fragment referencing an SVG document, it sets the initial value of the given custom property to the given <declaration-value>. (By default, the initial value of a custom property is an invalid value.)

2.1. Setting via the CSS url() Function

When referencing an SVG image 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-color custom 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.

To accomodate this, this specification defines a new <url-modifier>, the param() function:

param() = param( <custom-property-name> <declaration-value> )

Note: You may recognize this as identical to the syntax of the param() fragment identifer.

Similarly to the param() fragment identifier, the param() <url-modifier> sets the initial value of the referenced custom property to the passed <declaration-value>. Also similarly to the fragment identifier, if multiple param() functions reference the same custom property, the last one wins.

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:
.foo {
  background-image: url("" param(--color var(--primary-color)));

3. Using SVG Parameters

Within an SVG image, any SVG parameters set the initial value of the corresponding custom properties. They can then be used directly in any CSS property via the CSS var() function.

For example, if an SVG image wanted to expose a --color parameter, it could use it like:
  <g style="fill: var(--color);">
    <path d="..." />

Can it be used in presentation attributes?

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.
  1. On each var() function, provide a fallback value, like fill: var(--color, blue).

  2. 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.

  3. 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.


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.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

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

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level 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.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at Questions should be directed to the mailing list.


Terms defined by this specification

Terms defined by reference


Normative References

Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. 14 January 2016. CR. URL:
Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. 20 February 2014. CR. URL:
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 14 August 2018. WD. URL:
Tab Atkins Jr.. CSS Custom Properties for Cascading Variables Module Level 1. 3 December 2015. CR. URL:
Elika Etemad; Tab Atkins Jr.. CSS Fill and Stroke Module Level 3. 13 April 2017. WD. URL:
Anne van Kesteren; et al. HTML Standard. Living Standard. URL:
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL:
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. 2 February 2018. WD. URL:
Anne van Kesteren. URL Standard. Living Standard. URL:

Informative References

Tab Atkins Jr.; Chris Lilley. CSS Color Module Level 4. 5 July 2016. WD. URL:

Issues Index

Can it be used in presentation attributes?