SVG id attribute not validated.

[MewPurPur]

https://svgwg.org/svg2-draft/struct.html#Core.attrib

So the id attribute should only work with strings that are a non-zero length sequence of ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF] | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]

The id attribute is meant to go by NmToken rules. But instead, it works for everything, even empty strings. Example:

<svg viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg"><linearGradient id="" x2="1"><stop stop-color="#fac"/></linearGradient><rect width="16" height="16" fill="url(#)"/></svg>

[MewPurPur]

Okay, let's see.

User agents should process "id" values in SVG files irrespective of XML validity.

So we only need to abide by the following:

The "id" attribute must be unique within the node tree, must not be an empty string, and must not contain any whitespace characters.

So we're good with id's for now.