diff --git a/spec.txt b/spec.txt
@@ -1350,8 +1350,8 @@ Closing code fences cannot have info strings:
An [HTML block tag](#html-block-tag) <a id="html-block-tag"></a> is
an [open tag](#open-tag) or [closing tag](#closing-tag) whose tag
name is one of the following (case-insensitive):
-`article`, `header`, `aside`, `hgroup`, `blockquote`, `hr`, `body`,
-`li`, `map`, `button`, `object`, `canvas`, `ol`, `caption`,
+`article`, `header`, `aside`, `hgroup`, `blockquote`, `hr`, `iframe`,
+`body`, `li`, `map`, `button`, `object`, `canvas`, `ol`, `caption`,
`output`, `col`, `p`, `colgroup`, `pre`, `dd`, `progress`, `div`,
`section`, `dl`, `table`, `td`, `dt`, `tbody`, `embed`, `textarea`,
`fieldset`, `tfoot`, `figcaption`, `th`, `figure`, `thead`, `footer`,