Definitely a bug in one form or another. The spec says:

 "Computed value: For <length> the absolute value, otherwise a percentage"

which none of the implementations actually seem to adhere to... I guess we should go with resolving percentage values and get the spec fixed as well, since that seems like the most common behavior (need to check what IE/Edge does, but they are at worst the odd one out.)

Also note that for Firefox (and bleeding edge/TP Safari) you'd want to add 'transform-box: fill-box' to ensure the "right" reference box is used (you seem to assume fill-box at least, hence "right".)

That's true. Per Firefox, the transform-origin should be relative to view-box by default for an SVG element. And thus, both Chrome's rendering and computed values are incorrect.

Yes, 'transform-box' is currently behind a flag (ref  issue 595829 .)

Also, it's odd that `transform-origin: 0% 0%` and `transform-origin: 0 0` produce different rendering. Can anyone explain this?

That is also related to transform-box - or rather the lack of it - percentage values will be relative to the bounding box of the element while non-percentage will be relative to the origin of the current coordinate system.

But what will the right (per-spec) behavior once `transform-box` is available?

With 'transform-box', 'transform-origin: 0% 0%' will be the same as 'transform-origin: 0 0' (regardless of the value of 'transform-box'.)

fs@ any idea what the web-platform-test situation is like here?  Maybe we should at least try to land a couple tests around SVG and transforms?

After the merge of the CSS testsuites to WPT, there should be a fair amount of tests at least: https://github.com/w3c/web-platform-tests/tree/master/css/css-transforms-1 - the "bubble" in the draft claims 951 tests for the whole CSS transforms spec. Because of the mass of tests I've only sifted through them... I've yet to find coverage for computed style and percentage values (and SVG.) There's definitely tests for 'transforms' and SVG, but to what degree they are up-to-date and correct I don't know.

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I'd guess it's still important to address given spec compatibility.

Possibly related: https://bugs.chromium.org/p/chromium/issues/detail?id=915426

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cf11cdf7a29d2d73ed0de5472749f2b87bd924e6

commit cf11cdf7a29d2d73ed0de5472749f2b87bd924e6
Author: Fredrik Söderquist <fs@opera.com>
Date: Tue Dec 18 17:30:36 2018

Resolve computed 'transform-origin' for SVG elements

Make the local helper ReferenceBoxForTransform available via
ComputedStyleUtils and use it in TransformOrigin::CSSValueFromComp...
Replace usages of MinimumValueForLength with FloatValueForLength. This
matches what is done when resolving/applying transform in the general
case.
Also sink the common handling of the 'z' component out of the 'if'.

Bug:  740300 
Change-Id: Id0e6cbcc85942ccc8cdb3f8ed86ecc6e7aaf492e
Reviewed-on: https://chromium-review.googlesource.com/c/1382434
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617548}
[modify] https://crrev.com/cf11cdf7a29d2d73ed0de5472749f2b87bd924e6/third_party/blink/renderer/core/css/properties/computed_style_utils.cc
[modify] https://crrev.com/cf11cdf7a29d2d73ed0de5472749f2b87bd924e6/third_party/blink/renderer/core/css/properties/computed_style_utils.h
[modify] https://crrev.com/cf11cdf7a29d2d73ed0de5472749f2b87bd924e6/third_party/blink/renderer/core/css/properties/longhands/transform_origin_custom.cc
[add] https://crrev.com/cf11cdf7a29d2d73ed0de5472749f2b87bd924e6/third_party/blink/web_tests/external/wpt/css/css-transforms/transform-origin-014.html