Review path-length syntax

[chrishtr]

The SVGWG resolved to add a new path-length CSS property to apply to paths in SVG. This issue tracks the CSSWG reviewing the syntax and making any necessary changes.

[tabatkins]

All the SVG geometry attributes (which take unitless numbers in their value) which upgraded to presentation attributes express their property value with lengths (or length-percentage). Just as the very first example of such a property I found in the index, the cx property (for the center of a <circle>) takes <length-percentage> (while the cx="" attribute takes a number).

path-length isn't special in any way here. It's defining a length, which other properties (stroke-dash-array, etc) refer to. Its grammar should be <length>. (I don't think there's anything that <percentage> could be relative to, so probably not useful here.)

[kbabbitt]

@viralipurbey can correct me if I'm wrong, but I think the reason path-length takes a <number> is that it's applied as a scaling factor. https://w3c.github.io/svgwg/svg2-draft/paths.html#PathLengthAttribute:

The author's computation of the total length of the path, in user units. This value is used to calibrate the user agent's own distance-along-a-path calculations with that of the author. The user agent will scale all distance-along-a-path computations by the ratio of ‘pathLength’ to the user agent's own computed value for total path length.

(emphasis mine)

[viralipurbey]

@kbabbitt yes, that's correct. pathLength is the author's defined total path length, and the UA uses it to scale distance-along-a-path computations by the ratio of pathLength to its own computed value.

That said, I agree with Tab that it should use <length>. Even though it's used to derive a scaling ratio, the value itself is a length, and this follows the standard SVG attribute (<number>) → CSS property (<length>) pattern. The SVG WG was also aligned on making it <length> (minutes (https://www.w3.org/2026/03/26-svg-minutes.html)).

<percentage> doesn't make sense here though, no reference value to resolve against.

[tabatkins]

Yup, @viralipurbey is correct. "Applied as a scale factor" isn't particularly relevant here. It's specified as a length. A scale factor is then calculated from that, in concert with the UA's calculated length. (By dividing two lengths to form a dimensionless scale factor! The units check out. ^_^)

[kbabbitt]

Thanks, I misunderstood how the value was applied, makes sense to me now.

[kbabbitt]

Proposed resolution: the CSSWG thinks path-length should accept <length>

[tabatkins]

And specifically: only <length>, like the other attributes that only took numbers (representing SVG length units) and then were promoted to CSS properties (where the "SVG length unit" is the CSS px).

[css-meeting-bot]

The CSS Working Group just discussed Review path-length syntax, and agreed to the following:

• RESOLVED: path-length CSS property based on the SVG attribute will take <length> only

The full IRC log of that discussion

<JoshT> kbabbitt: SVG has the path-length attr
<JoshT> ... represents the authors computation of total path length
<iank_> (belated regrets from me sorry. If we can delay the replaced intrinsic sizing issue #6 on current agenda that'd be appreciated)
<JoshT> ... UA divides by its length of the path to make a ratio for things like text on a path
<JoshT> ... SVGWG resolved to add a path-length CSS property that works the same way. works same as other geometry properties
<JoshT> ... syntax is <number> using SVG user units
<JoshT> ... with consistency with others upgraded to CSS properties, we think it should take only a <length>
<JoshT> ... proposal is that we think path length--
<TabAtkins> yup that's it
<JoshT> ... path-length CSS property should take <length>
<JoshT> JoshT: and not including <number>?
<JoshT> astearns: SVGWG OK with a unitless number?
<JoshT> TabAtkins: they've been OK with that in other upgraded properties
<astearns> s/OK with/OK without/
<JoshT> PROPOSED: path-length CSS property based on the SVG attribute will take <length> only
<JoshT> RESOLVED: path-length CSS property based on the SVG attribute will take <length> only