ox-html: Provide a default CSS class for embedded SVG images

* lisp/ox-html.el (org-html--svg-image): Set CSS class of embedded SVG
images to be org-svg if no other class is set in attributes.
org-html-style-default: Provide default properties for new CSS class.

* doc/org.texi (CSS support): Documentation of default CSS class
org-svg for SVG images embedded into exported HTML.

doc/org.texi

@@ -11750,6 +11750,7 @@ div.footnotes       @r{footnote section headline}
 p.footnote          @r{footnote definition paragraph, containing a footnote}
 .footref            @r{a footnote reference number (always a <sup>)}
 .footnum            @r{footnote number in footnote definition (always <sup>)}
+.org-svg            @r{default class for a linked @file{.svg} image}
 @end example
 
 @vindex org-html-style-default

etc/ORG-NEWS

@@ -208,6 +208,11 @@ point for the SRC/EXAMPLE block.
 With the global variable ~org-latex-images-centered~ or the local
 attribute ~:center~ it is now possible to center an image in LaTeX
 export.
+**** Default CSS class ~org-svg~ for SVG images in HTML export
+SVG images exported in HTML are now by default assigned a CSS class
+~org-svg~ if no CSS class is specified with the ~:class~ attribute. By
+default, the CSS styling of class ~org-svg~ specifies an image width
+of 90\thinsp{}% of the container the image.
 *** Babel
 **** Support for SLY in Lisp blocks
 See ~org-babel-lisp-eval-fn~ to activate it.

lisp/ox-html.el

@@ -440,6 +440,7 @@ for the JavaScript code in this tag.
     { font-size: 10px; font-weight: bold; white-space: nowrap; }
   .org-info-js_search-highlight
     { background-color: #ffff00; color: #000000; font-weight: bold; }
+  .org-svg { width: 90%; }
   /*]]>*/-->
 </style>"
   "The default style specification for exported HTML files.
@@ -1628,21 +1629,27 @@ a communication channel."
      info)))
 
 (defun org-html--svg-image (source attributes info)
-  "Return \"object\" appropriate for embedding svg file SOURCE
-with assoicated ATTRIBUTES. INFO is a plist used as a
-communication channel.
+  "Return \"object\" embedding svg file SOURCE with given ATTRIBUTES.
+INFO is a plist used as a communication channel.
 
-The special attribute \"fallback\" can be used to specify a fallback
-image file to use if the object embedding is not supported."
+The special attribute \"fallback\" can be used to specify a
+fallback image file to use if the object embedding is not
+supported.  CSS class \"org-svg\" is assigned as the class of the
+object unless a different class is specified with an attribute."
   (let ((fallback (plist-get attributes :fallback))
 	(attrs (org-html--make-attribute-string
-		(plist-put attributes :fallback nil))))
-  (format "<object type=\"image/svg+xml\" data=\"%s\" %s>\n%s</object>"
-	  source attrs
-	  (if fallback
-	      (org-html-close-tag
-	       "img" (format "src=\"%s\" %s" fallback attrs) info)
-	    "Sorry, your browser does not support SVG."))))
+		(org-combine-plists
+                 ;; Remove fallback attribute, which is not meant to
+                 ;; appear directly in the attributes string, and
+                 ;; provide a default class if none is set.
+                 '(:class "org-svg") attributes '(:fallback nil)))))
+    (format "<object type=\"image/svg+xml\" data=\"%s\" %s>\n%s</object>"
+	    source
+	    attrs
+	    (if fallback
+		(org-html-close-tag
+		 "img" (format "src=\"%s\" %s" fallback attrs) info)
+	      "Sorry, your browser does not support SVG."))))
 
 (defun org-html--textarea-block (element)
   "Transcode ELEMENT into a textarea block.