With HTML export and org-export-with-planning t, no translation is done in org-html-planning to convert the angle brackets of an active timestamp to < and >. This gets shown by some browsers anyway, but is swallowed and ignored by others.

Expected behavior: active timestamps have characters properly turned into HTML entities.

Actual behavior: active timestamps retain angle brackets and therefore the HTML produced does not validate.

Suggested remedy: use org-html-plain-text to convert the output of org-timestamp-translate to HTML.

Emacs  : GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Package: Org-mode version 8.3.4 (8.3.4-28-gf9464d-elpa @ c:/Users/sut89614/AppData/Roaming/.emacs.d/elpa/org-20160328/)

--
Keith Sutton