References: href, xlink and url() · SVG file format

An SVG file is a graph: elements point at other elements by id. The explorer’s Resources tab has a References table with every pointer, where it leads and whether the target exists and is of the right kind, and an Ids table with how often each id is used.

The three kinds of address

Written as	Where it points
`#id`	An element of this document. The id is percent-decoded first.
`file.svg`, `file.svg#id`, `https://…`	Another document. Not loaded when the SVG is shown as an image, and often blocked for `use` by the same-origin rule.
`data:image/png;base64,…`	A file inside this one. The explorer lists each embedded image, decodes it and lets you open it as a file of its own.

href and xlink:href

SVG 1.1 writes links as xlink:href with the XLink namespace (xmlns:xlink="http://www.w3.org/1999/xlink"). SVG 2 uses a plain href and deprecates the XLink form, which still works everywhere and is still needed by old software. When both are present href wins.

Element	href points at
`use`	Any element to copy, not one that contains the `use`.
`a`	An address to open, or a view.
`image`, `feImage`	A picture file; `feImage` may name an element.
`linearGradient`, `radialGradient`	Another gradient to inherit stops and attributes from.
`pattern`	Another pattern.
`filter`	Another filter (SVG 1.1).
`textPath`	A `path` (SVG 2: any basic shape).
`mpath`	A `path` for `animateMotion`.
`animate`, `set`, `animateMotion`, `animateTransform`, `discard`	The element to animate, when not the parent.
`script`	A script file.
`tref` (1.1)	The element whose text to copy.

url() references in properties

Property	Must point at
`fill`, `stroke`	`linearGradient`, `radialGradient` or `pattern`. A fallback after the address (`url(#g) red`) is used if the server is missing; without one, a missing reference is an error and the shape is not painted (SVG 1.1) or painted with `none` (SVG 2).
`clip-path`	`clipPath`
`mask`	`mask`
`filter`	`filter`. A missing filter turns the element off in browsers.
`marker-start`, `marker-mid`, `marker-end`	`marker`
`cursor`	`cursor` (SVG 1.1) or an image address

The same references can sit in style attributes and in style sheets (.a { fill: url(#g) }); the explorer follows those too. The address may be quoted: url("#g").

Other kinds of name

in and in2 on filter primitives name a result of an earlier primitive in the same filter, or SourceGraphic, SourceAlpha and the old BackgroundImage, BackgroundAlpha, FillPaint, StrokePaint.
begin and end of animations name another animation or element: begin="a.end+1s", begin="b.click".
aria-labelledby and aria-describedby name title and desc elements by id.
A URL fragment can name a view: file.svg#front.

What the explorer reports

an id that no element has, with the attribute and element that refer to it;
a reference to an element of the wrong kind (a fill pointing at a clipPath);
a use inside the element it draws;
an in that names a result no earlier primitive makes;
an id used by more than one element, or containing white space.

Not described

This page is partial.

Reference: the exact error handling of each element for a broken reference, cross-origin rules for use and image, and the #svgView() fragment syntax are not described in full.
Viewer: resolves references inside the file, checks their kind, and lists the others. It does not fetch addresses, follow references into other files, or check aria-labelledby targets; begin and end ids are checked only for the form id.event.