A file that renders one thing and declares the fields it accepts. The markup an editor fills in and the markup a page renders are the same file, so they cannot drift apart.
The examples: block in each partial's frontmatter. This page renders them live — nothing here is a screenshot or a copy of the real component.
Yes. A partial is a convenience, not a gate: hand-composed pages pass their own HTML and render exactly as written.
Filler so the stage behaves like a page — something to scroll, and something for a translucent bar to pass over. The component above is the subject; this is only the ground it stands on.
Filler so the stage behaves like a page — something to scroll, and something for a translucent bar to pass over. The component above is the subject; this is only the ground it stands on.