Support path-length CSS property for SVG path elements

[viralipurbey]

Expose the path-length CSS property for SVG path elements, promoting the existing pathLength presentation attribute to a CSS‑expressible path property as defined in SVG 2.

CSS values follow normal cascade and override the presentation attribute. No new web APIs or security surface are introduced; existing pathLength behavior is preserved.

Example

<svg width="200" height="60">
  <path id="p" d="M10 30 L190 30" pathLength="100" />
</svg>

<style>
  /* CSS overrides the presentation attribute */
  #p {
    path-length: 200;
    stroke: black;
    stroke-dasharray: 50 150;
  }
</style>

In this example, the path-length CSS property overrides the pathLength="100" attribute and participates in normal CSS cascading and animation.

[emilio]

The example above makes it seem like that property accepts quirks / unitless lengths. We should avoid introducing more of that imho. But seems reasonable otherwise.

[emilio]

Cc @longsonr

[martinthomson]

As a matter of procedure, we need more information about this proposal:

• Which venue is discussing this proposal?
• What is the URL of the specification (or proposed specification change)?

This is small, but it would be appreciated if the information in our position form were filled out, rather than providing the specification in an issue comment.

[viralipurbey]

The example above makes it seem like that property accepts quirks / unitless lengths. We should avoid introducing more of that imho. But seems reasonable otherwise.

Good point. 'path-length' is not a CSS . It is a dimensionless representing an abstract author-defined total length for the path's geometry. It scales the mapping between the actual geometric path length and the authored value, affecting 'stroke-dasharray'/'stroke-dashoffset' computations and 'textPath' positioning.

The property definition accepts none | <number [0,∞]>, not . So there's no quirks/unitless-length issue here, it's intentionally a plain number, consistent with how the SVG 'pathLength' attribute has always worked.

[viralipurbey]

As a matter of procedure, we need more information about this proposal:

• Which venue is discussing this proposal?
• What is the URL of the specification (or proposed specification change)?

This is small, but it would be appreciated if the information in our position form were filled out, rather than providing the specification in an issue comment.

Thanks for flagging. Here's the requested information:

Venue: SVG Working Group (W3C)
Specification: The 'pathLength' attribute is defined in SVG 2: https://svgwg.org/svg2-draft/paths.html#PathLengthAttribute
Proposed spec change: The SVG WG is discussing promoting 'pathLength' to a CSS property in w3c/svgwg#773. Formal CSS property text has not been drafted yet. This follows the same pattern as other SVG geometry attributes (r, cx, cy, d) that were promoted to CSS properties.

Apologies for not filling out the form completely, happy to update the issue description with the full template if that would be helpful.