In short, no. If you made a typo, reply to your comment with a correction. Put "Correction" in the subject line of your correction to let people know that the comment has been amended.

That's why there's a Preview button. Check that your post says what you want it to say before clicking Submit. Also remember that you generate a post for every click of the Submit button, so only click Submit once. If you do submit a comment, then realise that there's a mistake, the common way of correcting it is to reply to your post with an appropriate subject line, e.g. "Correction, 'had' should read 'had not'." You cannot edit or remove a post once it has been submitted. Moderators have the ability to moderate posts if they deem such action appropriate, but the moderated post can still be viewed by users who have "Show comments that have been moderated" ticked in their settings. In exceptional circumstances, posts can be deleted from the database altogether, but only a select few of the moderators can do this, but please note those moderators HATE doing this, it's tedious, horrible, and they will be bitter for days. They aren't going to do it to fix a simple typo.

A limited set of HTML commands are allowed in posts. You will see them listed under the textbox on the page where you compose a comment. These commands are:
<strong>, <pre>, <a href="">, <b>, <li>, <strike>, <i>, <sub>, <ol>, <code>, <p align="">, <blockquote>, <u>, <br>, <ul> and <sup>.
However, you do not need to use HTML in order to post. If you type your post with no HTML codes it will appear just as you typed it.

The most common mistake made in posts with HTML is missing line breaks. If you're just typing plain text into a comment, any line breaks you put in will be preserved. However, if you use any HTML at all - if you put a link in your post, or make some text bold, for example - the engine will assume your entire post is in HTML, and so you must put a <br> tag wherever you want a line break. If you don't do that, and your post contains HTML, line breaks in your post will be ignored.
The other tag to watch out for is the <PRE> tag. If used incorrectly it will cause problems with the display of the board. Again the common mistake is not including line breaks. If this happens your post may need to be moderated.
If you are unfamiliar with HTML you can use the preview function once you've typed your post to see if it displays the way you'd expect.

When you create your account, you are automatically allocated a diary space. You can get to your diary by following the "My Diary" link at the left hand side of the page, and you can get to another person's diary by clicking on their name next to any post they've made. Alternatively, you can go to http://ars.userfriendly.org/users/diary.cgi and type the name of the person whose diary you want. The content at the top of your diary is defined in your settings under "Diary top-level permanent content". You can change this bit as often as you like, and you can put HTML in it. Write a bit about yourself, or put your omnicode there. Note - this is not the same as making a top level post, sometimes referred to as a TLP or starting a thread. Only you can create top level posts in your diary, but anyone can reply to those posts. Therefore, nobody else can put anything in your diary until you've created at least one TLP. To create a top level post, go to your diary and click the "Post New Comment" link. Aside from the fact that only you can make top level posts, your diary works in much the same way as the main discussion board. If you are new to the board, create your diary and a top level post, so everyone else can come and introduce themselves, welcome you to the board, and generally clutter up your diary.

No, not in this case. While your diary is your own little space on the UF board to talk about what you want, anyone can view it. However, you are the only person who can edit the top of your diary ("permanent content") and only you can make TLPs."

The 14 year old sister rule and all other UF board posting rules apply to diaries, though there is a little more leeway. If a moderator sees something and thinks it violates the FYOSR, it will be moderated. No guarantees that a moderator will view your diary though, there's enough work to do. If there is something that was posted in your diary that violates the rules and that you want moderated, you can drop an email to one of the moderators and they will be happy to take a look and moderate if necessary.