I had to make sure I do my part to discourage a "Perpetual Thursday".
No I'm kidding, but I couldn't comment without looking at the site.
First, you might as well forget about getting a Blogger blog to validate as XHTML. Unless you can get Blogger to change the ampersand "&" in some of their URLS to & it's never going to validate. You'll keep getting errors or warnings that say somthing like this because of the missing "amp;".
&postID is not a valid entity
The link color on the background image as shown below is going to be hard to read for some visitors. My eyes had to work a little to adjust & I have a tendancy to use colors without enough contrast myself.
Actually, when you look at this screenshot you can see that even the JPEG processor had a hard time telling the difference in some areas.

I generally believe that menu or navigation sidebars should retain a fixed width no matter what size the window is, but that's personal preference.
/* As this is a single-page site, all the CSS is included in this page, instead of in an external file. */
Good thinking.
I'm guessing you haven't gotten around to combining some of the CSS rules yet.
font-family:Georgia, Times New Roman, Times, serif;
font-size:0.8em;
/* could be */
font: normal 0.8em Georgia,"Times New Roman",Times,serif;
- font-family:Georgia, Times New Roman, Times, serif;
- font-size:0.8em;
- /* could be */
- font: normal 0.8em Georgia,"Times New Roman",Times,serif;
I'm curious as to why you're using somthing like
<strong>Links (Non-Blogs)</strong><br />
As opposed to
<h6>Links (Non-Blogs)</h6>
Come to find out, there isn't a single <h1-6> element on the entire page.

I noticed you have a tendancy to use class attributes on elements with few or no non-text decendant elements.
This is needed in some cases, but if you can help it try to reserve using class attributes to container elements. It's easy to target decendant elements from there.
li.post div p a {
color: #654321;
}
li.post address small a {
color: #123456;
}
li.post address small {
display: block;
}
<li class="post">
<h2>Title</h2>
<div>
<p>Lorem <a href="#">Ipsum</a></p>
<p>Lorem <a href="#">Ipsum</a></p>
<p>Lorem <a href="#">Ipsum</a></p>
<p>Lorem <a href="#">Ipsum</a></p>
<p>Lorem <a href="#">Ipsum</a></p>
</div>
<address>
<small><em>Lorem</em> <a href="#">Ipsum</a> dolor</small>
<small><em>Lorem</em> dolor</small>
</address>
</li>
- li.post div p a {
- color: #654321;
- }
- li.post address small a {
- color: #123456;
- }
- li.post address small {
- display: block;
- }
- <li class="post">
- <h2>Title</h2>
- <div>
- <p>Lorem <a href="#">Ipsum</a></p>
- <p>Lorem <a href="#">Ipsum</a></p>
- <p>Lorem <a href="#">Ipsum</a></p>
- <p>Lorem <a href="#">Ipsum</a></p>
- <p>Lorem <a href="#">Ipsum</a></p>
- </div>
- <address>
- <small><em>Lorem</em> <a href="#">Ipsum</a> dolor</small>
- <small><em>Lorem</em> dolor</small>
- </address>
- </li>
Strong with this one, the sudo is.