Dear Xara...

Who is this Round and why does he object?
… it may be apocryphal but they say that Churchill asked for a report from some expert in the Second World War. As is the way of the civil service, it was seen and annotated by others before it reached the great man. In the margin against some particularly contentious point, someone had written ‘Bollocks!’ A more senior and discreet mandarin carefully inked the offending word out and substituted ‘Round objects!’ Against which Churchill in turn asked, yes you’ve got there before me, ‘Who is this Round and why does he object?’
Round Objects.xar

Bug - Your HTML filter transforms my 256 x 256 px sized Rounded Rectangles with varying Line Widths into SPAN tags, each with absolute border-radius: 128px.
You should be checking the Curvature of the rectangle, squaring that and expressing the result as a percentage.
Here all are Curvature 0.707. Squared gives 0.5, so border-radius: 50% is the required CSS code. [a border-radius greater than 50% has the same appearance]
If you must return an absolute value it would have to be border-radius: 192px for a 256px circle. The maths for this would be onerous.

Can you please implement this change.

Acorn