Guidelines for writing microcopy: the small bits of text in the user interface.

• Page titles
• Navigation menu items
• Subheadings and tab titles
• Table column headers
• Form labels, instructions, and formatting hints
• Error messages
• Success messages
• Loading messages
• Empty states ("nothing added yet", "no results found")
• Buttons

Note that the Documentation Interest Group has a brief style guide in place.

• Buttons
• Icons
• Tables and grids
• Tab sets
• Multi-step processes
• Cards
• Trees
• Modals, confirmations, and dialogs
  • Button alignment
  • Focus order and keyboard controls
• Search forms
• Edit forms
• Progress bars
• Banners and alerts
• Date pickers
• Pagination
• Comboboxes and select lists

• Record detail + related items
• Introductory pages (splash screens)

• Color
• Typography
• Spacing
• Visual hierarchy