CSS guides

This first batch publishes 3 of the 20 upstream CSS guides; the rest are converted and will be released in later batches.

• Animate to Intrinsic Sizes — Smoothly animate interactive components (like accordions, menus, and expanding cards) to and from their natural dimensions.
• Fluid Scaling — Scale items like font size, spacing, and media sizes smoothly based on the parent container’s size rather than using fixed breakpoints
• Style a Form Group When a Field Is Invalid with :has() — Style parent elements of a form field (e.g. labels or fieldsets) when the field is invalid.