You can use most HTML tags in topics without a problem. This is useful where you want to
add some content that is formatted in a way that is not supported using WikiSyntax, for example,
you can write deleted text.
There are a few usability and technical considerations to keep in mind: 
Script tags may be filtered out, at the discretion of your Wiki administrator.
Recommendations when pasting HTML from other sources (using the plain-text editor):
<strike>deleted text</strike> to get - On collaboration pages, it's better not to use HTML, but to use WikiSyntax instead - this keeps the text uncluttered and easy to edit using the plaintext editor.
- If you must use HTML, use XHTML 1.0 Transitional syntax.
- Use
<literal>..</literal>tags around blocks of HTML to avoid accidental interpretation of WikiSyntax within the HTML.
Script tags may be filtered out, at the discretion of your Wiki administrator.- Copy only text between
<body>and</body>tags. - Remove all empty lines. Foswiki inserts
<p />paragraph tags on empty lines, which causes problems if done between HTML tags that do not allow paragraph tags, like for example between table tags. - Remove leading spaces. Foswiki might interpret some text as lists.
- Do not span a tag over more than one line. Foswiki requires that the opening and closing angle brackets -
<...>- of a HTML tag are on the same line, or the tag will be broken. - In your HTML editing program, save without hard line breaks on text wrap.