#+HTML_HEAD_EXTRA: <style> .org-svg { width: 20vw; } </style>

* test case
  #+BEGIN_SRC asymptote :file svg-image.svg
    size (1cm, 0);

    fill (unitsquare, red);
  #+END_SRC

  #+ATTR_HTML: :fallback fallback-image.jpg
  #+RESULTS:
  [[file:svg-image.svg]]

* generation of fallback image
  #+BEGIN_SRC asymptote :file fallback-image.jpg
    size (2cm, 0);
    fill (unitcircle, lightblue + opacity (.5));
    label ("fallback", (0, 0));
  #+END_SRC