私はこれがうまくいくと思います:
<div style="padding:20px;">
<div id="pagecontent">
<span class="main-content-font">
The title of this page goes in the gray box above. This is the homepage, you can put <u>anything</u> here (the main content of your website
which has some neat features and explains what your site is about should go here)!<br />
<br>
Content, content, and more content!<br />
<br>
Try to make it fill up as much space as possible, making the page longer. Don't fill it with useless junk, just anything
you can think of that will benefit the page.
</span>
<span class="whatever">
some things
</span>
</div>
</div>
試したことはありませんがmain-content-font
、スパンを作成しても改行が追加されないため、whatever
スパンはその右側に配置されます。