Sentence case buttons, and keep them short but clear, using no more than 3 or 4 words. The reader should know what’s going to happen after they click.
Button copy in newsletter and feature announcement emails can be a little less direct, but should be thematically appropriate. E.g. "Get flexible" for the flexbox launch email.
No matter the deliverable, buttons should begin with a verb in the imperative (command) form.
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Clear, direct, forceful, witty
Directed, confident, vaguely amused