(define-abbrev-table 'html-mode-abbrev-table
   '(("..." "…")   ; won't work
     ("---" "—")    ; won't work
     ("aaa" "…")   ; works
     ("bbb" "—") )) ; works

(require 'sgml-mode)
(modify-syntax-entry ?- "w" html-mode-syntax-table)
(modify-syntax-entry ?. "w" html-mode-syntax-table)