Q. 1 I put my SVG files on the Web in compress form (i.e. .svgz), but browsers don't seem to be able to view them but want the viewer to save the file.
A. Your web server needs to tell the requester that the file is compressed SVG. It does this by returning the correct Content-Type in the header of the HTTP response. You need to tell your web server about these files. Add 
AddType image/svg+xml .svg .svgz
AddEncoding x-gzip .svgz
to the <VirtualHost> stanza in your httpd.conf file. (See http://httpd.apache.org/docs/1.3/mod/mod_mime.html.) Alternatively, you can add it to an .htaccess within a directory to have it apply to it and all sub-directories. That directory must be configured in the server to allow .htaccess, i.e. the server has to know to read the .htaccess file. Note that the server must return a Content-Encoding of x-gzip and not gzip for Firefox to understand the type correctly. Opera understands both forms.
Q. 2 I've included the AddType and AddEncoding directives in the Apache configuration. However, things still don't work.
A. Make certain to clear your cache. Your browser is still probably I use 
library(RCurl)
x = getURLContent(u, .opts = list(verbose = TRUE), binary = TRUE)
to look at the header of the response to see what is happening. You can also use the Live HTTP Headers extension for Firefox.
Q. 3 I can read .svgz files from the Web but not locally.
A. It is not clear that Firefox has support for this yet. Opera does.
Q. 4 Tooltips. How do I get these to work for different browsers?
A. Within a path, for Opera, specify a title node as a child of path with the text of the tooltip. For Firefox, put a xlink:title attribute on the path. For Webkit (Safari and Google Chrome), enclose the path element within an a element with a xlink:title attribute.

We can refer to a CSS with a relative file name, a full path name, file:///Users/duncan/Rpackages/SVGAnnotation/CSS/RSVGPlot.css or file://localhost/Users/duncan/Rpackages/SVGAnnotation/CSS/RSVGPlot.css