It really would be more beneficial for you to keep the external CSS file. It saves so much more time when you want to make a quick update on the appearance of your site. Also, unless I have misunderstood what you are trying to do, CSS is what you will want to stick to if you would like to conform to XHTML standards, because most of the formatting tags (such as the bold, italic, underline, and center tags to name a few) have been dpreciated.
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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.