ul ... ⁄ul, optional element ⁄occurrences

The ul element is optional.
ul can be added once or multiple times per HTML5 document.
Examples

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

Valid Syntax 2
1 ul element present
1 2 3 4 5 6 7 8 9 10 11	<!doctype html> <html> <head> <title>...</title> </head> <body> <ul>...</ul> </body> </html>

Valid Syntax 3
multiple ul elements present
1 2 3 4 5 6 7 8 9 10 11 12 13	<!doctype html> <html> <head> <title>...</title> </head> <body> <ul>...</ul> <ul>...</ul> </body> </html>