When using standard DocBook XSL SVG icons for HTML output, how can the width be specified?

[Christopher BROWN <brown@reflexe.fr>]

Hello,

I'm using DocBook XSL 1.76.1 with Xalan and FOP, and would like to use
SVG icons for callout bugs.

Using "callout.graphics.extension", I can specify ".svg" and it
works... except that the icons ares HUGE, so that's not going to work.
 I looked for a "callout.graphics.width" parameter but it looks like
that's not provided for with the standard "html/callout.xsl" template
(the template "callout-bug" doesn't set the width or height
parameters).

Why am I trying to switch to SVG?  Because I now have to target HiDPI
screen users (read: MacBook Pro users with "Retina" screens) and the
upscaling of the PNG images looks pretty awful beside the "paper
quality" text.

Would it make sense to modify the XSL and add "callout.graphics.width"
as an XSL parameter?

Or will I just have to write some CSS rules, for example something like:

a.co > img, div.calloutlist a > img {
  width: 12px;
  height: 12px;
}

The second selector might map onto the wrong "img" tags though as it
doesn't have any class.  I suppose I could try my luck with
img[src*="/callouts/"] but I'm not sure I can try my luck with that
CSS3 selector yet...

Thanks,
Christopher