XHTML image tag mandatory alt attribute

The alt attribute is mandatory for the image tag, it is possible to have it written out with an empty string.
In case with the image tag there is no alt attribute the empty string alt attribute is added.

src/htmldocvisitor.cpp

@@ -126,21 +126,29 @@ static bool mustBeOutsideParagraph(DocNode *n)
   return FALSE;
 }
 
-static QString htmlAttribsToString(const HtmlAttribList &attribs)
+static QString htmlAttribsToString(const HtmlAttribList &attribs, const bool img_tag = FALSE)
 {
   QString result;
   HtmlAttribListIterator li(attribs);
   HtmlAttrib *att;
+  bool alt_set = FALSE;
+
   for (li.toFirst();(att=li.current());++li)
   {
     if (!att->value.isEmpty())  // ignore attribute without values as they
-                                // are not XHTML compliant
+                                // are not XHTML compliant, with the exception
+				// of the alt attribute with the img tag
     {
       result+=" ";
       result+=att->name;
       result+="=\""+convertToXML(att->value)+"\"";
+      if (att->name == "alt") alt_set = TRUE;
     }
   }
+  if (!alt_set && img_tag)
+  {
+      result+=" alt=\"\"";
+  }
   return result;
 }
 
@@ -1521,7 +1529,7 @@ void HtmlDocVisitor::visitPre(DocImage *img)
       else
       {
         m_t << "<img src=\"" << correctURL(url,img->relPath()) << "\""
-            << sizeAttribs << htmlAttribsToString(img->attribs())
+            << sizeAttribs << htmlAttribsToString(img->attribs(), TRUE)
             << "/>" << endl;
       }
     }