Why doesn't my slider look exactly the same in every browser/device?

The position of the layers, font sizes are calculated by javascripts. That javascript calculates as exactly as it can to make it look smooth when you make your slider smaller, not just to jump between sizes. For example your text's size can be 4.58036px, but it's up to the browser, if it shows the text with this exact number or rounds it up or down, and with the same value, it still shows it a little differently. The layers are calculated from the size of the slider too, and they can have the same issues with their positions and sizes.

The point is that there can be some pixel differences, and you shouldn't count on not having them.