Exactly what the percentage applies to depends on the tag. Percentages must be enclosed in quotes. This can be an absolute or a relative URL, depending on the situation. In most cases, both are permitted. It is recommended that URLs always be enclosed in quotes.
Any character is permitted, including entities.
It is recommended that strings are always enclosed in quotes. Each part gets a hexadecimal number between 00 and FF, and it should be given in two digits at all times.