It seems that Anki (I’m on Version 23.12.1 (1a1d4d54), Python 3.9.15 Qt 6.6.1 PyQt 6.6.1), SVG elements are ignored for determining if the card is empty or unique.
To reproduce, try to create a card with Front
<svg width="200" height="100">
<rect width="200" height="100" fill="blue" />
</svg>
raises the error shown in this screenshot:
Moreover, if one creates the two following cards:
- Front of card 1
What color is this?
<svg width="200" height="100">
<rect width="200" height="100" fill="blue" />
</svg>
- Front of card 2
What color is this?
<svg width="200" height="100">
<rect width="200" height="100" fill="red" />
</svg>
they are considered the same (duplicates).
Please see also this comment in a related AnkiDroid issue, with some more information: [BUG]: <svg> not displayed nor accepted as valid input · Issue #17440 · ankidroid/Anki-Android · GitHub