LiveJournal prohibits the direct use of several HTML tags: <iframe>, <embed>, and <object>, as well as some attributes of the <div> tag. Exceptions are made for the <iframe>, <embed>, and <object> tags when used to embed music, videos, or other active content into entries or profiles. Please note that the <iframe> tag can only be used in conjunction with sites that are on a site-wide whitelist of approved services. Using the <iframe> tag in conjunction with an unapproved service will result in no content being displayed.
In areas such as comments and customizations, you may need provide a text link to content you like to embed, rather than including it directly.