Do.
1. Cards have rounded corners.
2. Cards can have multiple actions.
3. Cards can be dismissible and rearranged.
Do.
A quickly scannable list, instead of cards, is an appropriate way to represent homogeneous content that doesn't have many actions.
Do.
Grid tiles are a clean and lightweight way to present a gallery of images.
Card content type and quantity can vary greatly. Cards within a card collection can each contain a unique data set. For example, various cards within a card collection might contain a checklist with an action, a note with an action, and a note with a photo.
Cards provide context and an entry point to more robust information and views. Don't overload cards with extraneous information or actions.
Content hierarchy
Use hierarchy within the card to direct users’ attention to the most important information. For example, place primary content at the top of the card, or use typography to emphasize primary content.
Images can help users easily compare and contrast content between cards. However, their size and placement within the card depends on whether images are the primary content or being used to supplement other content on the card.
Background images
Text is most legible when placed on a solid color background with a sufficient contrast ratio to the text. Text placed on image backgrounds should preserve text legibility.