org-e-html: Don't use default href for headlines linked via customid.

If headline is linked via customid in Org file, use customid in HTML
export as well.  Additionally, while linking to headline from within a
TOC section, use customid preferentially in HTML export.

contrib/lisp/org-e-html.el

@@ -1364,7 +1364,7 @@ that uses these same face definitions."
 		   text
 		   (and tags "   ") (org-e-html--tags tags))))
     (format "<a href=\"#%s\">%s</a>"
-	    headline-label
+	    (org-solidify-link-text headline-label)
 	    (if (not nil) headline
 	      (format "<span class=\"%s\">%s</span>" todo-type headline)))))
 
@@ -2101,8 +2101,9 @@ holding contextual information."
 	 (text (org-export-data (org-element-property :title headline) info))
 	 (tags (and (plist-get info :with-tags)
 		    (org-export-get-tags headline info)))
-	 (headline-label (concat "sec-" (mapconcat 'number-to-string
-						   headline-number "-")))
+	 (headline-label (or (org-element-property :custom-id headline)
+			     (concat "sec-" (mapconcat 'number-to-string
+						       headline-number "-"))))
 	 (format-function (cond
 			   ((functionp format-function) format-function)
 			   ((functionp org-e-html-format-headline-function)
@@ -2524,31 +2525,49 @@ INFO is a plist holding contextual information.  See
 			   (org-export-resolve-id-link link info))))
 	(case (org-element-type destination)
 	  ;; Fuzzy link points nowhere.
-	  ('nil
+	  ((nil)
 	   (format "<i>%s</i>"
 		   (or desc
 		       (org-export-data
 			(org-element-property :raw-link link) info))))
 	  ;; Fuzzy link points to an invisible target.
 	  (keyword nil)
-	  ;; LINK points to an headline.  If headlines are numbered
-	  ;; and the link has no description, display headline's
-	  ;; number.  Otherwise, display description or headline's
-	  ;; title.
+	  ;; Link points to an headline.
 	  (headline
-	   (let* ((headline-no (org-export-get-headline-number destination info))
-		  (label (format "sec-%s" (mapconcat 'number-to-string
-						     headline-no "-")))
-		  (section-no (mapconcat 'number-to-string headline-no ".")))
-	     (setq desc
-		   (cond
-		    (desc desc)
-		    ((plist-get info :section-numbers) section-no)
-		    (t (org-export-data
-			(org-element-property :title destination) info))))
-	     (format "<a href=\"#%s\">%s</a>" label desc)))
+	   (let ((href
+		  ;; What href to use?
+		  (cond
+		   ;; Case 1: Headline is linked via it's CUSTOM_ID
+		   ;; property.  Use CUSTOM_ID.
+		   ((string= type "custom-id")
+		    (org-element-property :custom-id destination))
+		   ;; Case 2: Headline is linked via it's ID property
+		   ;; or through other means.  Use the default href.
+		   ((member type '("id" "fuzzy"))
+		    (format "sec-%s"
+			    (mapconcat 'number-to-string
+				       (org-export-get-headline-number
+					destination info) "-")))
+		   (t (error "Shouldn't reach here"))))
+		 (desc
+		  ;; What description to use?
+		  (cond
+		   ;; Case 1: Link already has a description.  Use it.
+		   (desc desc)
+		   ;; Case 2: Link has no description and headline is
+		   ;; numbered. Use the section number.
+		   ((org-export-numbered-headline-p destination info)
+		    (mapconcat 'number-to-string
+			       (org-export-get-headline-number
+				destination info) "."))
+		   ;; Case 3: Link has no description and headline is
+		   ;; un-numbered.  Use the headline title.
+		   (t (org-export-data
+		       (org-element-property :title destination) info)))))
+	     (format "<a href=\"#%s\">%s</a>"
+		     (org-solidify-link-text href) desc)))
           ;; Fuzzy link points to a target.  Do as above.
-	  (otherwise
+	  (t
 	   (let ((path (org-export-solidify-link-text path)) number)
 	     (unless desc
 	       (setq number (cond