Greetings.

The attached patch
- adds a default CSS class org-svg to SVG images embedded in exported
  HTML if no other class is set for the image using attr_html
- documents the existence of this class.

The second attachment is an org-file that can be used to verify that the
patch works as expected. I have tested that it works in Safari, Firefox
and Chrome (Chromium).

If this patch is ok, then what is missing are the default CSS settings
for the new class org-svg in constant org-html-style-default. I simply
did not know what to put there. The manual says that this constant has
basic settings for _all_ defined CSS entities. Either defaults need to
be set for the new class, or the text in the manual needs to be
changed. Or we have to accept that the manual is not logically
coherent. :-)

Jarmo