A patch for ox-html to use CUTOM_IDs on id tags.

ox-html.patch

diff --git a/lisp/ox-html.el b/lisp/ox-html.el
index 757006321..0286317e5 100644
--- a/lisp/ox-html.el
+++ b/lisp/ox-html.el
@@ -2256,11 +2256,10 @@ line of code."
 		(format "<span class=\"linenr\">%s</span>"
 			(format num-fmt line-num)))
 	      ;; Transcoded src line.
-	      (format "<code%s>%s</code>"
-                      (if num-start
-                          (format " data-ox-html-linenr=\"%s\"" line-num)
-                        "")
-                      loc)
+	      (if num-start
+		  (format "<code data-ox-html-linenr=\"%s\">%s</code>"
+			  line-num loc)
+		loc)
 	      ;; Add label, if needed.
 	      (when (and ref retain-labels) (format " (%s)" ref))))
        ;; Mark transcoded line as an anchor, if needed.
@@ -2635,7 +2634,8 @@ holding contextual information."
           (format "<%s id=\"%s\" class=\"%s\">%s%s</%s>\n"
                   (org-html--container headline info)
                   (concat "outline-container-"
-			  (org-export-get-reference headline info))
+			  (or (org-element-property :CUSTOM_ID headline)
+			      (org-export-get-reference headline info)))
                   (concat (format "outline-%d" level)
                           (and extra-class " ")
                           extra-class)