summary ... ⁄summary, optional element ⁄occurrences

The summary element is optional.
1 summary element allowed per 1 details element.
Examples

Valid Syntax 1
summary element absent; optional element
1 2 3 4 5 6 7 8 9	<!doctype html> <html> <head> <title>...</title> </head> <body> </body> </html>

Valid Syntax 2
1 summary element present per 1 details element
1 2 3 4 5 6 7 8 9 10 11 12 13 14	<!doctype html> <html> <head> <title>...</title> </head> <body> <details> <summary>...</summary> ... </details> </body> </html>

Valid Syntax 3
2 summary elements per document; 1 summary per details element
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19	<!doctype html> <html> <head> <title>...</title> </head> <body> <details> <summary>...</summary> ... </details> <details> <summary>...</summary> ... </details> </body> </html>