Parent Templates
A parent template is recommended when you wish to assign peripheral styling to the content generated from the template. A single parent template can be applied to multiple regular templates. This means, that the CSS styling of one template can be applied to multiple template within Salesforce. This will ensure that your content is consistently styled.A parent template is just a template, with a CSS Static Resource specified for the styling.
A template body should have:
No SOQL query
No merge fields
The string “{0}” within the body of the template will indicate the spot that a regular template will be inserted.
If a template has a defined parent template and the parent template does not have ‘{0}’, then the results will only render the parent template and no content.