Why does the w3 validator say onClick is invalid?

Question

w3 validator gives me this error

there is no attribute "onClick"

I do not see what makes my code invalid.

<a href="#" onClick="return BookmarkApp.addBookmark(this)"><img alt="" src="./favorites-intro.jpg" /></a>

offers?

Answer 1

If you change it to lowercase onclick, the error will disappear. Like this:

<a href="#" onclick="return BookmarkApp.addBookmark(this)"><img alt="" src="./favorites-intro.jpg" /></a>