CSS if width of

I'm quite new to CSS (a day lol) and trying to set up something to test for width with Stylish. A script on the page is setting the width of a certain object, I'm trying to make an 'if width is equal/not equal to 0, apply these declarations' but I can't find anything that can do this. Closest I got was

.ytp-volume-panel *:not([width:0]) > .ytp-volume-slider-panel {foo=''}

for the not equal to 0 and I have no idea how to go about the equal to because there's no if. Am I overcomplicating this?