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Google Mail has four text sizes, Small, Normal, Large, and Huge. Avoid using Small as that can be difficult to read. Additionally keeping the default black text on a white background, or other higher contrast combinations, should be used for better readability. Use bold or underline to denote emphasis rather than using different color text.
Formatting an email signature block to be accessible
To format your email signature (sig) so that it remains accessible, please follow these steps: EMAIL: How to format your email signature with the Fredonia Logo