What characters are allowed in an HTML attribute name?

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It depends what you mean by “allowed”. Each tag has a fixed list of attribute names which are valid, and in html they are case insensitive. In one important sense, only these characters in the correct sequence are “allowed”.

Another way of looking at it, is what characters will browsers treat as a valid attribute name. The best advice here comes from the parser spec of HTML 5, which can be found here: https://html.spec.whatwg.org/multipage/syntax.html#attributes-2

It says that all characters except tab, line feed, form feed, space, solidus, greater than sign, quotation mark, apostrophe and equals sign will be treated as part of the attribute name. Personally, I wouldn’t attempt pushing the edge cases of this though.

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