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We use Helvetica, as this is the font that is recommended by the Federal Identity Program. Arial is provided as a backup as the Helvetica font is not as widely supported.
For accessibility, some users do better with serif fonts while others do better with sans-serif (much like some low vision users and users with colour deficiencies need high contrasting colours while users with light sensitivities need low contrast and darker backgrounds). There is no one font or colour combination that meets all accessibility needs.
Our policy is to provide reasonable defaults and to ensure users are able to override the default fonts and colours to meet their individual needs. If you prefer serif fonts, you could configure the accessibility settings in your browser to force the font that best meets your needs.