Problem 

​A client was recently upgraded from SharePoint 2010 to SharePoint 2013 and noticed that, by default, the Arial font option is no longer available in the font drop-down list of the "Format Text" tab in the ribbon when interacting with the rich text editor.  This is odd because it was present in SharePoint 2010.

SharePoint 2010

SharePoint 2013

At least they're sorted alphabetically now, right?  :-)

Resolution

First, refer to this post:  http://social.technet.microsoft.com/Forums/sharepoint/en-US/63d614a1-258d-4efc-a4c5-88b7f3422df3/adding-a-new-font-to-the-list-of-available-fonts?forum=sharepointadminprevious.

Following this post, I was able to get Arial added back to the list.  At the most basic level, I just added the following CSS to a Content Editor web part on one page.

<style type="text/css">
.ms-rteFontFace-20
{
     -ms-name: "Arial";
     font-family: "Arial";
}
</style>

Note:  The "20" of the "ms-rteFontFace-20" class name is to avoid interference with other already-defined such classes.  Increase/decrease the number as appropriate.  Higher numbers are likely safer.

While that will solve this problem for one page, you probably want it to affect an entire site collection, in which case you can:

1. Add the style to the site collection's default master page.

2. Create your own custom CSS file, add the style to it, and have your site collection's default master page reference your CSS file.

3. Create your own custom CSS file, add the style to it, turn on the site collection's Publishing Infrastructure feature, then reference your CSS file through the site's master page settings.

Summary