Not really, Firefox and Chrome supported the picture element well before Safari did. It was part of the nascent HTML5 standard.
I believe what WebKit introduced was the analogous CSS image-set property, which likely influenced the picture's element's development (but image-set still hasn't managed to escape draft-spec territory).
I believe what WebKit introduced was the analogous CSS image-set property, which likely influenced the picture's element's development (but image-set still hasn't managed to escape draft-spec territory).