The value for the animation-direction property determines how animations behave after they complete. To run animations in reverse order after completion, you should set animation-direction to reverse. This will play the animation backward.

To make a box 10% of the viewport width, you should use the 'vw' (viewport width) unit. The 'vw' unit is based on the width of the viewport, making it ideal for responsive design. It ensures the box is always a percentage of the viewport's width, regardless of the screen size.

SMACSS categorizes its styles by specificity. In SMACSS, styles are organized into five categories: Base, Layout, Module, State, and Theme. This categorization helps in creating a scalable and maintainable CSS architecture by defining how styles should be applied and structured.

For an element with position: sticky; to work, the parent container must have a defined overflow property (e.g., overflow: auto;). This allows the element to stick within the scrolling container when it reaches a certain point, behaving as if it's "stuck" to the viewport.

To make flex items stack vertically in a Flexbox container, you set flex-direction to column. This means the items will be arranged one below the other, creating a vertical stack.

To display list items side by side in a navigation menu, you should use the display: flex; property on the parent container of the list. This allows for a horizontal layout of the list items.

In responsive design, the term "adaptive" describes a layout that adjusts and looks good on all screen sizes and devices. An adaptive design ensures that the content and layout respond effectively to various screen sizes, providing a seamless user experience.

To create a loading spinner that rotates continuously, you need to use CSS animations. Key properties for this effect include animation-duration (to control the duration of the animation), animation-timing-function (to specify the timing function for animation), animation-iteration-count (to set it to 'infinite' for continuous rotation), and animation-direction (to make it rotate continuously using 'alternate' or 'normal').

CSS-in-JS solutions, such as styled-components or Emotion, can help prevent unused styles by generating scoped styles at runtime. This means only the styles required for a specific component are injected, leading to improved performance.

To separate styles that apply to layout from those that apply to modules, you should consider the ITCSS (Inverted Triangle CSS) methodology. ITCSS offers a scalable and logical structure for your stylesheets, with layers that guide you in distinguishing between core layout styles and module-specific styles, promoting maintainability and organization.