Hi,

I have been trying to export SVG images having links to CSS from Org to 
HTML
and I have found that the this CSS is not active in Firefox (only 
browser I tried).

I have found that the CCS is only active under two conditions:
1. The HTML page should manage the CSS that the SVG image links to.
2. The SVG image should be embedded in the HTML page.

I wrote an ox-html derived export backend to embed SVG images:
https://forge.chapril.org/gav451/emacs.d/src/branch/main/site-lisp/ox-my-html/ox-my-html.el
and merged the SVG embedding functionality into ox-html.el leading to 
the
attached patch.

I have included an MWE below and I attached mwe.org and mwe.html with
doc8.svg and style8.svg (showing a flower and chosen for the visual 
impact).

You will get an M-non-WE after exporting mwe.org using ox-html on the
main branch.

# begin MWE
#+title: SVG and CSS MWE with patch or with ox-my-html backend
#+HTML_DOCTYPE: html5
#+HTML_HEAD: <link rel="stylesheet" type="text/css" href="style8.css" />
#+OPTIONS: ^:{} toc:nil html-embed-svg:nil
# The next line overrules html-embed-svg:t
#+HTML_EMBED_SVG_EXCLUDES: ./doc8.svg
# The next line overrules html-embed-svg:nil
#+HTML_EMBED_SVG_INCLUDES: ./doc8.svg

[[https://developer.mozilla.org/en-US/docs/Web/SVG/Tutorial/SVG_and_CSS][SVG 
and CSS (MDN)]] is the origin of the files =doc8.svg= and
=style8.css=.  Open =doc8.svg= and =mwe.html= in Firefox to compare
hovering over the flower.  The flower is fully black in case of an
M-non-WE or in case of HTML export without patch or without ox-my-html
backend.

#+CAPTION: Is MWE with SVG embedding *and* with HTML_HEAD managing
#+CAPTION: the CSS. Becomes an M-non-WE without SVG embedding or
#+CAPTION: without HTML_HEAD managing the CSS.
[[./doc8.svg]]
# end MWE

Best regards -- Gerard