Here is the list of HTML tags and attributes currently allowed on PerlMonks. See PerlMonks Approved Chatter HTML Tags for what is allowed in the chatterbox. Currently home nodes do not have their HTML filtered, but a separate list of allowed HTML for home nodes is being developed.
|a||href, name, target, class, title, rel|
|blockquote||class, cite, lang, dir|
|col||align, col, span, width, class|
|colgroup||align, col, span, width, class|
|del||class, datetime, cite|
|font1||size, color, class|
|ins||class, datetime, cite|
|table||width, cellpadding, cellspacing, border, bgcolor, class|
|tbody||width, align, valign, colspan, rowspan, bgcolor, height, class|
|td||width, align, valign, colspan, rowspan, bgcolor, height, class|
|tfoot||width, align, valign, colspan, rowspan, bgcolor, height, class|
|th||width, align, valign, colspan, rowspan, bgcolor, height, class|
|thead||width, align, valign, colspan, rowspan, bgcolor, height, class|
|tr||width, align, valign, colspan, bgcolor, height, class|
1Although these tags are allowed, use of them is discouraged. In particular, it is usually much better to use <p> and not <br /> and please use <code> instead of <pre> unless the enclosed lines are pretty short.
2Note that <code> and <c> are not really HTML tags at PerlMonks. They prevent enclosed special characters like <, >, &, [, and ] from being interpretted. Also, not even whitespace is allowed inside (< code > will not be recognized) and they must have a matching </code> or </c> for them to work. If the code contains a newline, then it is rendered such that long lines will wrap.
The only string not allowed inside of <code> tags is </code> (though upper- vs. lower-case is ignored). Likewise, the only strings not allowed inside of <c> tags is </c> (again, upper- vs. lower-case is ignored).
3Note that <readmore> is not an HTML tag. The tag becomes a <div class="readmore"> when viewing the node directly (so it can be seen as a slight change in background color behind the surrounded text, depending on how you have CSS set up).
But, when viewing the node as part of some other node (such as a reply seen when viewing a node earlier in the thread or such as a root node seen in The Monastery Gates or in a section display page like Seekers of Perl Wisdom), it replaces the enclosed content with <br />Read more...<br /> where "Read more..." is a link to the node. Clicking that link shows you the node directly which makes the previously hidden content appear.
Also, not even whitespace is allowed inside (< readmore > will not be recognized) and, if there is no closing tag, then the rest of the node is affected.
You can use <code> to display a literal <readmore> (as one would expect, though it previously did not always work).
4Note that <spoiler> is not an HTML tag. It is used for presenting "spoilers" to questions and will be rendered differently according to the users preferences, with the general idea being that a little more work is required to see the content than just viewing a node.
When wrapped in <code> tags the <spoiler> tags will be treated as literal text and not have any special meaning