"'" => "'",
The apos entity is an XML built it, and isn't defined for HTML. While some browsers support it in text/html documents, this is error correction and you should not use it.
It's best to escape the data as it's coming in; otherwise it's very difficult to distinguish between, for example, a less-than sign that should be converted to < and one that is part of the markup.
My preference is to convert from text to HTML at the last minute to avoid issues where I need to manipulate the data in Perl. (Template::Stash::EscapeHTML is quite cool).
What matters though is doing it in one place, so its easy to spot when you forget to protect a bit of user input from XSS et al.
Are you posting in the right place? Check out Where do I post X? to know for sure.
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link
or How to display code and escape characters
are good places to start.