See https://www.mediawiki.org/wiki/Manual:Collapsible_elements.
The default toggle text is Expand/Collapse.
| The Header | remains visible |
|---|---|
| This content | is hidden |
| at first | load time |
If one wishes something different, data-collapsetext= and data-expandtext= may be set as in this example:
| The Header | remains visible |
|---|---|
| This content | is hidden |
| at first | load time |