User:Pedro Kakuse/Developers’ reference

ID attribute values
ID’s must begin with an uppercase or lowercase letter and may be followed by any combination of letters, numbers, hyphens, underscores, colons, and periods.

Character references
Word joiner (U+2060)  prevents wrapping between characters.

Soft hyphen (U+00AD)  enables wrapping and creates a visible hyphen.

Selectors
Identifiers (elements, classes, id’s) may only contain uppercase letters, lowercase letters, numbers, hyphens, and underscores. Identifiers may not begin with a number, two hyphens, or a hyphen followed by a number.

Wikipedia
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