[svgwg] Issue: Behaviour of `:percent` spatial media fragment in SVG when no natural size is given for the external image (#1113) marked as Agenda+

goldenboy777 has just labeled an issue for https://github.com/w3c/svgwg as "Agenda+":

== Behaviour of `:percent` spatial media fragment in SVG when no natural size is given for the external image ==
## Problem 
The [SVG 2 linking spec](https://svgwg.org/svg2-draft/linking.html#LinksIntoSVG) defers spatial media fragments (`#xywh=..`.) to the [Media Fragments 1.0 spec](https://www.w3.org/TR/media-frags/). [Media Fragments §4.2.1](https://www.w3.org/TR/media-frags/#naming-space) allows the `xywh` value to be expressed in either `pixel:` (default) or `percent:` units, where percent units "are interpreted relative to the size of the resource."

For raster media (image/video) the resource always has an intrinsic pixel size, so resolving percentages is well-defined. SVG is different: an `<svg>` document may have no intrinsic width/height (no width/height attributes and no viewBox), in which case it has no resolvable natural size until it is embedded in a context that supplies a concrete object size (CSS, img, object, etc.).

Neither spec defines what `xywh=percent:..` should resolve against in that case. 

## Implementations behaviour
I checked the behaviour of `:pixel` and `:percent` spatial fragments with the presence/absence of width, height or viewbox in the external svg. Below are my observations:

**1. Chrome:** Does not implement media fragments so external svg renders as is.
**2. Firefox**: when width and height are explicitly given expected square is rendered. But when width and height is not given a [fallback ](https://searchfox.org/firefox-main/source/dom/svg/SVGFragmentIdentifier.cpp#182)of 300x150 seems to be used for the resolution.

## Possible resolutions

1. **Reject the fragment**, treat the percent fragment as invalid when no natural size is available, fall back to the default view.
2. **Resolve against a default 300×150** (the CSS replaced-element default). (Firefox current implementation)

## Rendering Results

### Firefox

<img width="1410" height="1054" alt="Image" src="https://github.com/user-attachments/assets/4dc841d3-5311-40ad-9994-e22fa9f7daec" />

### Chrome

<img width="1667" height="1220" alt="Image" src="https://github.com/user-attachments/assets/fb28a0f1-93ab-4f94-8a03-9b8e93a9fd54" />

[spatial-media-fragments.html](https://github.com/user-attachments/files/28106222/spatial-media-fragments.html)

See https://github.com/w3c/svgwg/issues/1113


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Received on Thursday, 28 May 2026 12:52:13 UTC