SVG Parameters

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).

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.

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.

.foo {
  background-image: url("http://example.com/image.svg" 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.

<svg>
  <g style="fill: var(--color);">
    <path d="..." />
  </g>
</svg>

Can it be used in presentation attributes?