Created attachment 282344 [details] Test case Elements that have an inline style with an important rule cannot be updated via JavaScript. See attachment: by clicking on the button, the element on the page should change background-color AND width, but since the width inline style has the "!important" rule, it does not happen. I checked other main browsers, Chrome and Firefox change the style, while IE (and Edge) don't change it, just like Safari (and WebKit).
I'm happy to report that Bug 159959 (https://bugs.webkit.org/show_bug.cgi?id=159959) resolves the issue. Found this out in WebKit Nightly r203480.