SVG export rejects valid rgba paint with separate presentation opacity #288

Current-main reproduction

On d1376371965f513d99cc9ec388835d255c5c88d5, <rect fill="rgba(255,0,0,0.5)" fill-opacity="0.5" .../> renders red at alpha 64/255.

Save this controlled source as rgba-opacity.html:

<!doctype html><html><head></head><body style="margin:0"><svg id="original" width="40" height="40" viewBox="0 0 40 40" xmlns="http://www.w3.org/2000/svg"><title>Screen</title><desc>Controlled color square.</desc><rect fill="rgba(255,0,0,0.5)" fill-opacity="0.5" width="40" height="40"/></svg></body></html>

Run the packaged public helper:

python3 skills/diagram-design/scripts/export_svg.py rgba-opacity.html rgba-opacity.svg

It returns 2 with duplicate attribute because normalization adds a second fill-opacity. No SVG is written.

Expected behavior

references/export.md describes SVG export as carrying diagram CSS, prefixing defs IDs and rewriting matching local references, and lossless rgba normalization. A portable export should preserve the diagram's paint semantics.

Native controls

Verified with a browser rendering the owned source HTML and exported SVG, plus Pillow RGBA pixel comparison. A plain red presentation-attribute square still exports identically. The reproduction uses no source scripts, external assets, or network requests.

Proposed narrow scope

Normalize the existing paint/opacity combination without duplicate attributes or changing the browser appearance. Cover both fill and stroke, omission controls, and unrelated text that looks like an attribute.

No examples/templates, manifest versions, new diagram type, source rendering in the extractors, or source execution are involved.

Codex is assisting @rudycelekli with reproduction and implementation.