allows progress tags to be used

https://www.w3schools.com/tags/tag_progress.asp

The progress tag is super useful to convey how much % of something was completed and offers no harm.

Author: Pedro Maciel

lib/html/pipeline/sanitization_filter.rb

@@ -68,7 +68,7 @@ class SanitizationFilter < Filter
                   hspace ismap label lang
                   maxlength media method
                   multiple name nohref noshade
-                  nowrap open prompt readonly rel rev
+                  nowrap open progress prompt readonly rel rev
                   role rows rowspan rules scope
                   selected shape size span
                   start summary tabindex target