onclick or ONCLICK or onClick?

HTML is case-insensitive and therefore the casing of event handler names in your HTML documents is irrelevant. E.g. you can use onclick, ONCLICK, onClick or even oNcLiCk for your event handler name. However, XHTML is not so liberal and stipulates that all event handler names should be lowercase (e.g. onclick=”…”). Therefore, to make your HTML pages XHTML compliant, always use lowercase event handler names.