The font used in the HTML editor has been changed during the recent redesign. Depending on the fonts you have available on your machine, the font used will be either Consolas, Monaco, or your browser's default monospaced font.
Monospaced fonts are traditionally used when editing source code (such as HTML). According to Wikipedia:
Even though computers can now display a wide variety of fonts, almost every commercial IDE and software text editor employs a monospaced font as the default typeface. This increases the readability of source code, which is often heavily reliant on distinctions involving individual symbols
The visual editor should use either Georgia, Times New Roman, Bitstream Charter, Times, or your browser's default serif font - again, depending on the fonts available on your machine.