Go to settings > Writing and select “WordPress should correct invalidly nested XHTML automatically” and then click “Save Changes.”
The probelem could result form importing content, or from copying and pasting text containing invalid HTML without using icon 5 o 6 in Row 2 of the Visual editor.
Go to your latest post, open it in the editor, make one minor change such as adding a space and then deleting it, and then click “Update Post.” Check your blog and see if it is back to normal.
If it’s not, go to your latest post, open it in the editor and stwitch to the HTML editor, then look for an open HTML tag and close it. Check your blog again and see if it is back to normal.
The template underlying your theme is coded to provide a specific font family or style. The options for changing fonts in wordpress.com blogs are contained here > http://onecoolsitebloggingtips.com/2010/02/04/changing-fonts-in-wordpress-com-blogs/