http://w3c.github.io/html/syntax.html#the-in-body-insertion-mode the user agent must handle the token as follows:
...
An end tag whose tag name is "sarcasm"
Take a deep breath, then act as described in the "any other end tag" entry below.
Наверно даже, если интерпретировать rules as written, парсер, который не вздыхает на тэге - non-conforming.
(украдено из комментов)