Astro
Slots
Slots is the Astro name for placeholders in a component that parent content fills. A default slot takes everything, and named slots take specific pieces.
How it is measured
Put <slot /> in the component template. The parent's child content appears there. Add name="aside" to make a named slot, and fill it with slot="aside" on an element. Fallback content inside the slot tag shows when the parent gives nothing.
Check by rendering the component with and without children. The built HTML should show either your content or the fallback.
Worked example
A Layout component has a default slot for the page body and a named slot called sidebar. A glossary page fills both. A contact page fills only the body, and the sidebar falls back to a small help note.
A developer passes an h1 inside the sidebar slot by mistake and the page gets two h1 tags. The audit tool flags it right away.
How it differs
Slots pass markup into a component. Astro.props passes data. A slot is for content the parent writes; a prop is for a value the component reads.
Common errors
Using a slot name that does not exist and seeing nothing. Expecting slots to work the same inside framework components. Forgetting the fallback. Passing too much through one default slot. Mixing prop strings with slot markup.
In practice
Use one default slot for the main body and a few named slots for regions that vary. Always supply fallback content for optional areas. Keep names short.