Do not use Buttons as navigational elements → use Links.
In most cases, do not use more than 3 words for button labels.
Choose labels which are short and meaningful.
Avoid using acronyms as labels in buttons. Use language which the user will understand.

Accessibility compliance

Follow the Accessibility guidelines, especially:

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Unless otherwise specified, see our general compliance information in Fundamentals - Accessibility

Design

As per each individual button type.

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