The new fonts option can be seen in a Fonts section when you set up a new ad. For your campaign you can choose your account default (which is configured on your Ad Display Preference page), or the standard AdSense font-family, or any of the three Arial, Times and Verdana.
Note because the ad font will usually be displayed as text in the browser, it depends on the font existing on the client machine, though the three fonts you can currently pick from are very popular web-wide. Something else to keep in mind: some fonts, using the same font size definitions, will appear larger (Verdana) or smaller (Times New Roman) in comparison to Arial.
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