What is supported

• path (all commands, relative and absolute, arcs), line, polyline, polygon, rect (with corner radius), circle, ellipse
• stroke-linecap: butt, round, square (including dots for zero-length subpaths)
• stroke-linejoin: miter (with stroke-miterlimit), round, bevel
• inherited presentation attributes and inline style, display:none, visibility:hidden, defs and other non-rendered containers
• transform on any element, including non-uniform scale (the stroke width is scaled by the geometric mean of the matrix)
• filled shapes with nonzero and evenodd fill rules
• <use> (also into <symbol> and <defs>), and <style> sheets with simple selectors (tag, .class, #id, tag.class, comma lists)
• nested <svg> viewports and <symbol> viewBoxes, with x, y, width, height (percentages too) and preserveAspectRatio

Not supported yet

text, image, dashes, markers, clip paths, masks, filters, vector-effect, opacity, CSS combinators. Colours aren't kept either: everything ends up as one currentColor path.

None of these fail silently. When the input uses one of them, the conversion still runs and reports a warning through onWarning, or throws if you pass strict: true. On the command line that's a line on stderr, or a failed file with --strict.